1. Immunoblot experiments in rat prostatic epithelium using a non-selective antibody against protein kinase C (PKC) allowed to detect three PKC subspecies of 87.5, 55.5 and 34.6 kDa that showed higher, similar and lower immunoreactivity in the membrane than in the cytosolic compartment, respectively. 2. Specific monoclonal antisera revealed that the PKC-gamma isozyme is not expressed in the rat prostatic epithelium, whereas the PKC-beta isozyme was noted only in the cytosolic fraction showing an apparent molecular weight of 75.5 kDa. 3. Induction of diabetes by streptozotocin led to modifications in the expression of PKC isozymes so that the immunoreactivities of the 87.5- and 55.5-kDa PKC forms decreased in both cytosolic and membrane subcellular fractions to different extents. 4. The most important decrease was that of the 55.5-kDa PKC form in cytosol that returned to control values by insulin therapy, whereas PKC-beta suffered also some decrease in diabetes and increased again with insulin treatment.