The metabolic reactions in every organism are strongly related to the enzyme activity and energy input. The only source of energy in living cells is ATP (adenosine triphosphate). The substrate for energy metabolism enters the organism through food. For the same processes, body reserves such as glycogen, fat and proteins, can also be used. Energy needs are not always the same. Highly-productive animals, especially dairy cows, have high energy demands immediately after delivery due to increased milk production (lactogenesis). After birth, calf is physically separated from the placenta as a source of components needed for growth and development and is dependent on milk. For cow, delivery is energetically the most demanding period, where biochemical processes will use components from the digestive system, as well as storage depot for the purpose of milk synthesis. Using the trends of specific blood metabolites concentration, energy metabolism can be estimated with great precision. Indeed, the shifting of biochemical reactions to pathological state, makes biochemical blood tests useful for diagnostic purposes. Energy metabolism in cows and calves during early puerperium and early neonatal period were investigated by monitoring of biochemical parameters in blood. The study was conducted on cows (N=13) and their calves (N=13) of the Simmental breed. Blood samples were taken by jugular vein puncture (v. jugularis externa) at 6, 12 and 48 hours and 7, 14 and 30 days after calving. Serum samples were assayed for glucose, total protein, albumin, urea, triacylglycerols, total cholesterol, high density lipoprotein cholesterol (HDL cholesterol) and low density lipoprotein cholesterol (LDL cholesterol), β hydroxybutirate acid (BHB), non esterified fatty acids (NEFA), insulin and insulin like grow facotor I (IGF-I) concentrations. Results showed a significant decrease of serum glucose concentration of cows during the study periods accompanied by an increase in serum glucose of calves compared to the beginning of the study (P <0.05). Concentration of total proteins in the serum of cows was uniform throughout the experimental period, while the concentration of albumin showed a significant decrease from the 7th day onward (P <0.05). At the same time, the concentration of total proteins in serum of calves was significantly increased up to the 7 day after birth, while concentration of serum albumin was significantly increased from the 14 day of the research period (P <0.05). Concentration of urea in serum of cows was significantly decreased to 48 hour after birth (P <0.05). The urea concentration in serum of calves was significantly increased on the 7 day (P<0.05) and then significantly decreased on the 30 day after birth (P<0.05). The concentration of triglycerides in the serum of the cows were significantly reduced 48 hours after birth (P <0.05) and than was significantly increased at 7 day after birth up to the value recorded at the beginning of the experiment (P <0.05). The concentration of triglycerides in the serum of calves showed no significant difference during the experimental period. Concentration of total cholesterol in the serum of cows showed a significant increase from the 7 day after birth until the end of the investigated period (P<0.05). In calves, the concentration of total cholesterol has been significantly increasing during the study (P<0.05). Concentration of HDL cholesterol in the serum of cows showed a significant decrease up to 48 hour after birth, unlike 30 day after birth, when HDL cholesterol showed significant increase (P<0.05). The concentration of LDL cholesterol in serum of cows was significantly increased from the 7 until 30 day after birth (P<0.05). The concentration of LDL cholesterol in serum of calves was significantly increased from 12 hour after birth until the end of the study period (P<0.05). Concentration of BHB in serum of cows and calves were uniform during the study. Concentration of NEFA in the serum of cows was uniformed up to 14 day after birth, while at 30 day after birth concentration showed significant decrease compared to 48 hour after birth (P<0.05). In the serum of calves NEFA concentration was significantly lower starting at the beginning of the study period until the 14 day after birth (P<0.05), followed by a mild increase up to 30 day after birth. Serum insulin concentration was significantly decreased at the 7 and 14 day after birth compared to the concentration recorded at the beginning of the study (P<0.05). Insulin concentration in serum of calves showed no significant difference during investigated periods. The concentration of IGF-I in the serum of cows showed no significant differences. Concentration of IGF-1 in serum of calves showed a significant decrease (P<0.05) at 12 hour after birth followed by a significant increase 48 hour after birth and than stayed on similar values until 14 day after birth. The results of the investigated parameters in the blood serum of cows and their calves indicate rapid and significant changes in energy metabolism in the first hours after birth. The early neonatal period mainly differs in biochemical parameters compared to the 7 or 14 day, which is attributed to more intense energy metabolism and higher substrate requirements at a later stage.