Protein kinase C activity is present in rat prostatic epithelial cells in both cytosolic and membrane subcellular fractions. Partial purification by ion-exchange chromatography and characterization of cofactor requirements showed its behavior as a classical Ca(2+)- and phospholipid-dependent enzyme activated by 1,2-diacylglycerol (or by mimicking agents such as tumor-promoting phorbol esters). Streptozotocin-induced diabetes resulted in an increase of the membrane/cytosolic enzyme ratio suggesting a redistribution of protein kinase C from the cytosolic to the membrane fraction (an index of enzyme activation) that could be reversed toward control conditions by insulin treatment. Differences observed in cofactor requirements for maximal enzyme activation argue for a some distinct expression of protein kinase C isozymes in control and diabetic conditions. These results are a new aspect of the complex set of alterations exhibited by the diabetic prostate in the signal transduction mechanisms that mediate cell functions and proliferation in this gland which could be related to the prostate atrophy and impaired fertility characteristic of this disease.