1. Pre-incubation (1-5 min) with 12-O-tetradecanoylphorbol-13-acetate (TPA, 8-16 nM), a tumour-promoting phorbol ester known to activate protein kinase C, was found to inhibit acetylcholine-induced Ca2+-dependent K+ and Cl- currents in cells isolated from rat lacrimal glands. 2. Previous work showed that the same currents may be elicited by dialysing cells with a high-Ca2+ (1.0 microM) solution, with inositol trisphosphate (InsP3), or with guanosine 5'-[gamma-thio]triphosphate (GTP-gamma-S). 3. After TPA incubation both types of currents could be elicited by dialysis with elevated Ca2+ solutions, although the magnitude of the K+ current was slightly reduced in comparison with control cells. 4. Responses to intracellular dialysis with InsP3 (20 microM) were similar to those in untreated cells, indicating that the Ca2+ release process was unaffected. 5. However, the response to GTP-gamma-S (0.2-0.5 mM) dialysis was strongly inhibited in TPA-treated cells. 6. These results suggest that protein kinase C exerts an inhibitory action on the pathway leading from receptor activation to inositol trisphosphate production.