Cyclic AMP dependence of protein kinase activity (PrK) in testicular homogenates varies according to the substrate used (histone > casein). The enzymatic activity increased during maturation, notably between 35 and 45 days of age. The increment is higher in isolated seminiferous tubules than in interstitial tissue. Upon centrifugation at pH 4.8, two dissimilar PrK preparations can be obtained. The enzymatic activity of both fractions increased with maturation.