The superior cervical ganglion of rats was perfused with Ringer solution containing hexamethonium to produce a steady, partial, nicotinic block. The compound action potential (CAP) evoked by supramaximal single shock stimulation of the cervical sympathetic trunk (CST) was recorded from the internal carotid nerve. Bolus injection of the protein kinase C (PKC) activators 4 beta-phorbol-12,13-dibutyrate (PDBu) or 4 beta-phorbol-12,13-diacetate (PDAc) produced a marked, prolonged, dose-dependent potentiation of the CAP amplitude (e.g. 90% decay 2 h). A non-PKC activating phorbol ester (PE), 4 alpha-phorbol-12,13-didecanoate, produced no potentiation. The PE-induced potentiation was antagonized by the PKC inhibitor H-7. In addition, after 1 h exposure to PDBu (3 microM) and recovery from the potentiation (e.g. 2-4 h), a second exposure to PDBu or PDAc produced no potentiation. A 5 s 40 Hz supramaximal train to the CST produced a long lasting potentiation of the CAP (long-term potentiation, LTP) as described previously. However, a similar train did not evoke LTP after perfusion for 1 h with PDBu. The train-evoked LTP was depressed by the PKC inhibitor H-7 at a concentration which antagonized the PE-evoked potentiation. These data suggest that (i) PKC activation potentiates nicotinic transmission, and (ii) a component of the train-evoked LTP is mediated by PKC.