Protein kinase C (PKC) is a heterogeneous family of ten or more isoforms which plays an important role in neuronal signal transduction. Isoforms from all subclasses are prominently expressed in the rat hippocampus, as demonstrated by immunoblot with isozyme-specific antisera: Ca(2+)-dependent (alpha, beta I, beta II and gamma), Ca(2+)-independent (delta, epsilon and a newly characterized PKC related to eta) and atypical (zeta). In addition, the zeta isoform is also found as the free, constitutively active catalytic domain, protein kinase M zeta (PKM zeta). Two distinct patterns of expression of PKC isozymes in rat hippocampus are found during development from E18 to P28. PKC zeta, PKM zeta and PKC delta are present at birth and their expression does not increase postnatally. In contrast, the other isoforms are expressed only at low levels at birth and then increase in the first 4 weeks postnatally. These two patterns of expression suggest distinct functions for PKC isozymes during development.