Carnitine acetyl (CAT) and carnitine palmitoyltransferase (CPT) activities were determined in skeletal muscle, heart, liver and brown fat of fetal and adult monkeys. CAT activity was highest in brown adipose tissue at all ages, but there was no significant change with age. Muscle CAT activity was lowest and also not related to age. In both heart and liver, however, there was a significant (p < 0.01) increase with age. CPT activity, on the other hand, did not differ from organ to organ or with age. In brown fat from newborn dead human babies CAT activity was between 100 and 270 nmol/mg protein/min, much higher than in a spontaneously aborted 1,500-gram fetus (25 nmol). The highest CAT activity in newborns was found in muscle.