The cellular and regional distribution of the four protein kinase C (PKC) isoforms in the rat hippocampal formation and the response of PKC to lesions were determined by employing immunohistochemical and immunochemical techniques with antibodies specific to PKC(alpha), -(beta I), -(beta II), and -(gamma). PKC(alpha) intensely stained the periphery of the pyramidal cell in the stratum pyramidale. The granule cells, glial cells, and mossy fibers were anti-PKC(alpha) negative. The cytoplasm, axons, and dendrites of basket cells and interneurons in the hilus were labeled with anti-PKC(alpha). Anti-PKC(beta I) immunoreactivity was localized on the periphery of pyramidal cells and interneurons of the hilus, as well as the oriens, radiatum, and molecular layers of the CA regions. Anti-PKC(beta II) immunoreactivity was mainly cytoplasmic, extending into the dendrites in the hippocampal pyramidal cells and the dentate granule cells, and also in some glial cells. In the stratum radiatum of the CA1, anti-PKC(gamma) immunoreactivity localized to the pyramidal cell cytoplasm, extending into the dendrites. Following fimbria-fornix (FF) lesions, the anti-PKC(alpha) and -(beta I) staining of the pyramidal cell periphery was markedly reduced. The anti-PKC(gamma) staining of the pyramidal and granular cells of the dentate gyrus was reduced whereas the interneuron staining in the hilus was increased. In the FF-lesioned hippocampus, anti-PKC(alpha) and anti-PKC(beta II) labeled reactive glial cells, whereas anti-PKC(beta I) and -(gamma) did not. Quantitative Western blot analysis revealed a dramatic increase in the particulate/total PKC for all isozyme forms, although the total levels of PKC, except PKC(gamma), did not change following FF lesions. The PKC(gamma) concentration doubled after FF lesions. Perforant path lesions resulted in a marked alteration in the neuronal staining in dentate gyrus with anti-PKC(alpha) and -(beta I) and in increased numbers of anti-PKC(alpha)- and anti-PKC(beta II)-positive glial cells. Anti-PKC(gamma) staining did not change noticeably. The total PKC concentration did not change for isozymes alpha, beta I, and gamma, but PKC (beta II) concentration increased by 48% following perforant path lesions as detected by Western blot analysis. The particulate/total PKC decreased for all four isozymes although the reduction in PKC(beta I) concentration was not statistically significant. This change in PKC compartmentalization is in marked contrast to an increased level of particulate PKC following FF lesions. Thus, the effects of deafferentation and deafferentation for each PKC isoform were different.