3. The sensitivity of 1P3 recovers after short-term administration of ACh (0 2 ,tM), and in turn, the ACh-induced response is augmented by the presence of internal IP3. These results suggest that a synergism between IP3 and another ACh-induced substance plays an important role in muscarinic Ca2+ signalling.4. ACh-induced responses are inhibited by pre-incubation (10 min) with an activator of protein kinase C, TPA (12-O-tetradecanoylphorbol-13-acetate, 16 nM), or augmented by pre-incubation (10 min) with an inhibitor, H-7 (1-(5-isoquinolinesulphonyl)-2-methylpiperazine, 10 /tM), whilst IP3-induced responses are unaffected by that with both agents. These results indicate that protein kinase C acts negatively on the signalling elements prior to the formation of IP3.5. The oscillatory responses, induced by cell dialysis with a nominally Ca2+-free (ca 1-10 /tM) solution containing GTPyS (100 aM), are unaffected by the pre-treatment with TPA or H-7. In addition, these responses and/or those triggered by shortterm stimulation with ACh and internal GTPyS are not influenced by external ACh. On the other hand, the oscillatory responses recorded in acinar cells pre-treated with H-7 are tightly controlled by external ACh. 6. Taken together these results suggest that activation of protein kinase C does not affect the activity of GTP-binding protein, but disconnects the link between the muscarinic ACh receptor and GTP-binding protein, or inhibits ACh binding to the receptor, in rat pancreatic acinar cells.