Phorbol esters have been shown to enhance the release of transmitters and to potentiate the effect of local anaesthetics in olfactory cortex of the rat. This work examined the interaction between phorbol dibutyrate and a range of substances, which act on axonal conduction and synaptic transmission. Synaptically-evoked field responses were elicited by stimulation of the lateral olfactory tract of slices of olfactory cortex, maintained in vitro. Ketamine (0.2-1.0 mM), benzocaine (0.2-1 mM), atropine (0.1-2 mM) and tetrodotoxin (20-200 nM) depressed synaptic transmission and, in the presence of phorbol dibutyrate (1 microM), these substances were more potent by 3.7 +/- 0.5, 1.5 +/- 4, 2.6 +/- 0.6 and 5.5 +/- 1.8 fold, respectively. Pentobarbitone (0.1-2.0 mM) with or without bicuculline, urethane (10-200 mM), halothane (0.5-5.0 mM) (with bicuculline) and ethanol (50-500 mM) also depressed synaptic transmission but their effectiveness was not potentiated by phorbol dibutyrate. It is thought that the increased potency, produced by phorbol ester, was associated with a presynaptic action of those substances.