it has been suggested that the maintenance of long term potentiation (ltp) in the hippocampal mossy fiber (mf) synapse involves a presynaptic mechanism that does not require the activation of protein kinase c (pkc), since this enzyme appears to be absent in the mf presynaptic terminals. in the present study the authors evaluated this proposal by directly comparing the metabolic properties of hippocampal mf synaptosomes and a conventional p2b synaptosomal preparation prepared from the same hippocampal tissue. protein kinase c-dependent histone phosphotransferase activity was found to be comparable in mf and p2b synaptosomes. western blot analysis was performed using antisera prepared against four of the pkc isoforms, and the results demonstrate that the b and y pkc isoforms are present in relatively equivalent amounts in these two subcellular fractions however, the cytosolic fraction derived from the hippocampal mf synaptosomes appeared to contain a greater amount of the pkc-e isoform when compared to the p2b synaptosomal preparation. four distinct endogenous substrates present in the mf synaptosomes are shown to be phosphorylated in response to pkc activation