Aim: The aim of controled reproduction is enhanced fertility. Knowledge of the factors and mechanisms involved in folliculogenesis and luteogenesis has resulted in development of controled reproduction system with timed artificial insemination, resynchronization of cows that did not concieve at first AI, and strategies for better embryo survival rate. This is achieved through pharmacologicaly induced methods of folliculogenesis, luteolysis and luteogenesis and resynchronization in due time, with the main goal of enhancing fertility and preventing subfertility. Ovsynch is a routine procedure in postpartal insemination programs and in resynchronization programs, which are used to shorten the period between inseminations. Since the middluteal phase of oestrous cycle is desired time to start the protocol, the best way to achieve this is to apply presynchronization protocols. This induction protocols precede Ovsynch, and thanks to them, better cycle synchronization is achieved. Methodology: The research was conduceted on one dairy farm in Croatia, on 60 cows divided in three equal groups, each comprised of 20 animals. The first group A was presynchronized with GnRh and PGF nine and two days before startin Ovsynch. The second group B was presynchronized with just one application of PGF two days before Ovsynch. The third group C served as control, and was not subjected to presynchronization. All cows were artificially inseminated at fixed time. 23 days after TAI, GnRH was applied to all cows, as a start of Ovsynch in resynchronization. Seven days later, all cows were checked on pregnancy with ultrasound. Those diagnosed as non-pregnant continued with Ovsynch and TAI. Blood samples were collected for analysis of hormones (progesteron, estradiol, luteinizing hormone) and several metabolic parameters (urea, creatinin, cholesterol, total protein, glucosis, ALT, AST, GGT). Also, body condition score, parity and lactation was analysed for all cows. Results: The results indicate that cows recieving presynchronisation had better pregnancy rate comparedAim: The aim of controled reproduction is enhanced fertility. Knowledge of the factors and mechanisms involved in folliculogenesis and luteogenesis has resulted in development of controled reproduction system with timed artificial insemination, resynchronization of cows that did not concieve at first AI, and strategies for better embryo survival rate. This is achieved through pharmacologicaly induced methods of folliculogenesis, luteolysis and luteogenesis and resynchronization in due time, with the main goal of enhancing fertility and preventing subfertility. Ovsynch is a routine procedure in postpartal insemination programs and in resynchronization programs, which are used to shorten the period between inseminations. Since the middluteal phase of oestrous cycle is desired time to start the protocol, the best way to achieve this is to apply presynchronization protocols. This induction protocols precede Ovsynch, and thanks to them, better cycle synchronization is achieved. Methodology: The research was conduceted on one dairy farm in Croatia, on 60 cows divided in three equal groups, each comprised of 20 animals. The first group A was presynchronized with GnRh and PGF nine and two days before startin Ovsynch. The second group B was presynchronized with just one application of PGF two days before Ovsynch. The third group C served as control, and was not subjected to presynchronization. All cows were artificially inseminated at fixed time. 23 days after TAI, GnRH was applied to all cows, as a start of Ovsynch in resynchronization. Seven days later, all cows were checked on pregnancy with ultrasound. Those diagnosed as non-pregnant continued with Ovsynch and TAI. Blood samples were collected for analysis of hormones (progesteron, estradiol, luteinizing hormone) and several metabolic parameters (urea, creatinin, cholesterol, total protein, glucosis, ALT, AST, GGT). Also, body condition score, parity and lactation was analysed for all cows. Results: The results indicate that cows recieving presynchronisation had better pregnancy rate compared to cows that recieved Ovsynch only (A 29,41 %, B 47,37 % and C 21,05 %). In resynchronization, conception rate did not differ among groups, and was around 30 %. However, BCS, parity and lactation did not have any effect on pregnancy rate. Analysis of luteinizing hormone showed that all cows had ovulated. Progesteron concentration differed significantly between group C and groups A and B (p<0,001) on the start of Ovsynch in synchronization, thus indicating that combined protocols are more effective due to better homogenization of folliculogenesis and synchronization of ovulation. Conclusion: There is no doubt that the protocols that are based on Ovsynch (simple, and modified, but combined) represent a major step in understanding and managing the reproduction. The philosophy of this aspect is controlled induction of a new follicular wave and induction of ovulation synchronization. This is only a phase of research, one of which goals is a control of growth, function and duration of the dominant follicle, all in order to enhance fertility. Finally that would enhance milk production, reduce the morbidity and culling rate, and create a sufficient number of replacement heifers. Both of these two combined synchronization protocols used in this research are original, and they have not been previously used on dairy cow farms. Therefore, they are not found in up-to date literature. Due to good results, their application should be furtherly evaluated.