Leptin is mainly produced in adipose tissue and acts in the hypothalamus to regulate food intake. The objective of the present study was to examine the pattern of plasma leptin concentrations of calves during 45 days after birth and the effect of breed on leptin and insulin concentrations between Japanese Black and Holstein calves. Four Japanese Black and four Holstein calves were used. Animals were fed 10% of their body weight whole milk for a week after birth, and then milk replacer and commercial formula feed for growing calf at a level to meet the nutrient requirement. Body weights were measured on the day of birth and 45 days after birth. Blood samples were collected everyday for 1-6 days after birth, 2 days interval for 6-14 days after birth and 3 days interval 14-45 days after birth to determine plasma leptin and insulin concentrations. Body weight significantly (P < 0.05) increased as compared to day of birth at 45 days after birth. However, plasma leptin concentration slightly decreased (P < 0.05) as days after birth advanced throughout the experimental period. Plasma insulin concentration decreased (P < 0.05) drastically 3 days after birth in both breeds. Breed of the animals influenced on plasma leptin and insulin significantly (P < 0.05), and high values were observed in Japanese Black than in Holstein calves. It has been suggested that the rate of body fat contents in Japanese Black cattle is naturally higher than in Holstein cattle. Probably the high levels of plasma leptin and insulin in Japanese Black calves were affected by genetically body fat characteristics of each cattle, though it was not confirmed in present study.