The identification of multiple protein kinase C (PKC) cDNA sequences from rat, bovine, and human tissues has led to the discovery of a family of PKC genes. Using probes complementary to three cDNA sequences from rat brain, designated PKC-I, -II, and -III, the distribution of PKC transcripts in rat tissues was studied by in situ hybridization histochemistry. In brain, PKC-II and PKC-III transcripts colocalized except in the brain stem and spinal cord, where some cells appeared to contain only PKC-III message. PKC-I mRNA had a different, but partially overlapping, pattern of expression. In spleen, PKC-II and PKC-III, but not PKC-I, transcripts were concentrated in the marginal zone of white pulp, the major B-lymphocyte compartment of rat spleen. The distinct distributions of rat PKC mRNAs suggest that different members of the PKC family have specialized functions.