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Ontogenic changes of protein kinase C in the rat liver, heart and cerebrum were examined from 17-day gestation until adult. Cerebral protein kinase C activity was 19 times less at 17-day fetal as compared to 15-day postnatum or older rats. The enzyme activity in the heart was generally higher in the neonatal period than the adult, but was not correlated with the previously reported alpha 1-adrenergic and cholinergic receptor ontogeny. Likewise in the liver, the enzyme activity was not correlated with previously reported alpha 1-adrenergic and vasopressin receptor ontogeny. No correlation between protein kinase C and cyclic AMP-dependent protein kinase activities was found among these tissues. The enzyme distribution expressed by a cytosol/particulate ratio of 0.72, 2.41 and 0.64 in the liver, heart and brain, respectively, in 20-day fetus was similar to adult values. We conclude that there is a discrete ontogenic pattern of protein kinase C in each organ and it does not seem to be correlated with the ontogenic pattern of hormone receptors which relate to phosphatidylinositol breakdown. The precise role of protein kinase C in relationship to growth and differentiation needs to be further investigated.