Protein kinase C (PKC) is a Ca2+-dependent enzyme involved in synaptic transmission, which can be experimentally activated by the phorbol ester, phorbol 12-myristate-13-acetate (TPA). We studied the effects of TPA application on acetylcholine (ACh) release at the rat neuromuscular junction by means of the focal recording technique; possible effects of TPA at the postsynaptic site had been ruled out in preliminary studies. In extracellular solutions containing 2 mM Ca2+ and at the stimulation frequency of 0.1 Hz, TPA increased endplate current (EPC) amplitude. In non-stimulated preparations spontaneous current frequency was increased at a similar rate. The similar time course of TPA action on evoked and spontaneous currents suggests that an increased presynaptic Ca2+ efficacy can be considered to be the probable mechanism of action. The interactions of PKC with ACh release were further investigated. In 0.1 mM Ca2+ extracellular solutions, TPA enhanced evoked currents only at stimulation frequencies (e.g. 40 Hz) that were themselves capable of inducing facilitation. This facilitation is classically associated with presynaptic Ca2+ accumulation, indicating that PKC interacts synergistically with Ca2+ to facilitate ACh release. In particular, since mean quantum size and release probability remained almost unchanged during TPA facilitation, it was concluded that PKC acted by enlarging the immediately available store. Interestingly, TPA also increased the presynaptic currents that were observed to be largely brought about by Ca2+-dependent K+ currents: evidence was obtained to suggest that increases in these currents provide negative feedback against excess release activation rather than being an expression of enhanced Ca2+ influx.