Serotonin (5HT) is a biogenic amine present both in the brain (central compartment) and in peripheral tissues (peripheral compartment) where it has different physiological functions. In this thesis we have studied the influence of perinatal increase in serotonin concentrations on survival rate, weight gain and anxiety-like behavior in rat pups. The animals were treated with serotonin precursor 5-hydroxytryptophan (5-HTP) or with serotonin catabolism inhibitor tranylcypromine (TCP) from gestational day 13 to post-natal day 21. Control group was treated with saline. Both experimental groups had significantly lower survival rates, while the survived pups had significantly lower body mass and weight gain compared to the control group. Anxiety-like behavior was determined by analyzing ultrasonic vocalization after separation from litter on post-natal days 13, 16 and 18. TCP-treated animals had significantly reduced latency to the first call and significantly higher number of calls, number of call sequences and duration of calls. In animals treated with 5HTP, these values were similar to those of the control group. A delay in frequency transition from infantile (40 kHz) to juvenile (50 kHz) was observed in both experimental groups. The results suggest that exposure to increased serotonin concentrations during perinatal period negatively affects rat development, increases the level of anxiety and delays maturation. The effect of TCP, which raises serotonin level in both 5HT compartments, is more drastic than the effect of 5HTP, which raises serotonin concentration only in the peripheral compartment.