The expression of the different protein kinase C (PKC) isozymes in mouse thymocytes was studied to determine if there is a correlation between isozyme expression and thymocyte phenotype. Expression of PKC isozymes in thymocyte subsets (distinguished by the CD4 or CD8 Ag) was determined by message amplification phenotyping. The expression of mRNA for PKC-alpha, -beta, -epsilon, and -zeta, but not -gamma or -delta isozymes, was detected in all of the unstimulated thymocyte subpopulations analyzed. Thus no differences in the pattern of PKC isozyme expression were found that could be correlated with thymocyte phenotype. However, it was noted that the levels of PKC mRNA expression were affected by different stimuli in unfractionated thymocytes. Whereas mRNA levels of PKC-alpha and -beta were down-regulated by PMA and ionomycin treatment, no significant changes were seen in the levels of PKC-epsilon mRNA with these agents. PKC-epsilon mRNA decreased in thymocytes exposed to Con A similar to what has been reported for PKC-epsilon protein. PKC-zeta mRNA was also down-regulated by PMA or ionomycin, and the combination of both compounds caused a more rapid and drastic effect. Finally, PKC-delta mRNA expression was induced transiently in thymocytes only after exposure to PMA or Con A, and this induction was inhibited by ionomycin treatment. These results indicate that message levels of specific isoforms of PKC are uniquely regulated and suggest an additional level of control of PKC activity in activated lymphocytes.