1. Protein kinase C (PKC) is a family of enzymes that is activated by diacylglycerol (DAG) following phospholipase (PL) C activation. Protein kinase C may also be activated by metabolites and arachidonic acid generated by breakdown of membrane phospholipids by PLD and PLA2, respectively. Subsequent to PKC activation, key protein substrates are phosphorylated, resulting in the facilitation of transmitter release. 2. Phorbol esters are compounds that mimic the actions of DAG on PKC and have been shown to facilitate stimulation-induced (S-I) transmitter release in rat brain. However, some phorbol esters that have a high affinity for PKC have no effect on transmitter release, whereas others with a lower affinity for PKC markedly elevate S-I transmitter release. 3. The structure and, more importantly, the lipophilicity of the phorbol esters determines their ability to access and activate the intraneuronal pools of PKC that are involved with transmitter release. In studies in which cell membranes were intact, phorbol esters did not display the characteristics expected based on their affinities for PKC in contrast with studies in disrupted synaptosomes. This supports the hypothesis that the membrane plays a critical role in determining the effects of phorbol esters on PKC. 4. B-50, a PKC substrate thought to be involved in transmitter release, also appears to be differentially phosphorylated by various phorbol esters. The effects on B-50 phosphorylation in intact synaptosomes, but not disrupted synaptosomes, are well correlated with the effects of phorbol esters on S-I transmitter release. 5. B-50 is colocalized with actin, which has also been suggested to play an important role in facilitating the movement of reserve pools of transmitter vesicles to the readily releasable state. Therefore, it is possible that the phosphorylation status of B-50 directly influences the organization of actin filaments, thereby allowing transmitter output to be sustained under high levels of stimulation.