At birth, nine infant male cynomolgus monkeys (Macaca fascicularis) weighed an average of 402 g and 16 infant female cynomolgus monkeys weighed an average of 362 g. Both female and male infants last 15-20 g by the fourth day after birth. Female and male infants showed a growth rate of approximately 4 and 5 g body wt/day, respectively, from 4 to 150 days of age. Caloric intake increased from approximately 140 cal/gkg body weight/day in both females and males on the first day after birth to peak values of 325 and 290 cal/kg body weight/day in females and males, respectively, and subsequently declined to about 200 cal/kg/day in males and 250 cal/kg/day for females. Total calories consumed per day increased from 53 cal/day in females and 68 cal/day in males to approximately 200 cal/day after 60 days of age in both females and males. Erythrocyte counts, packed cell volume, and hemoglobin values decreased from birth to 2 weeks of age, then stabilized. An increase in lymphocyte count, with a concomitant decrease in mature neutrophils, was observed from birth to 6 weeks of age. "Temper fits" and certain aggressive behavioral signs were observed as early as 3-4 days after birth, and infants began to develop social orders as early as the 30-40th day. Behavioral abnormalities frequently seen in infants reared in isolation were not observed.