The phospholipase C-mediated hydrolysis of phosphatidylcholine has been shown recently to be activated by a number of agonists. Muscarinic receptors, which trigger various signal transduction mechanisms including inhibition of adenylate cyclase through Gi, have been shown to be potent stimulants of this novel phospholipid degradative pathway. We demonstrate here, by exogenous addition of Bacillus cereus phosphatidylcholine-hydrolyzing phospholipase C, that phosphatidylcholine breakdown mimics the ability of carbachol to inhibit adenylate cyclase. This effect is sensitive to pertussis toxin and is entirely dependent on the presence of protein kinase C. This kinase is also required for the inhibition by carbachol of adenylate cyclase. These results suggest that the activation of phosphatidylcholine breakdown by phospholipase C may play an important role linking or favoring the coupling muscarinic receptors to Gi. Results presented here also show that phospholipase C-mediated hydrolysis of phosphoinositides by exogenous addition of Bacillus thuringiensis phosphoinositide-hydrolyzing phospholipase C does not affect adenylate cyclase, despite the fact that protein kinase C is translocated to an extent similar to that produced by the hydrolysis of phosphatidylcholine. According to the results shown here, both phospholipases also differ in their ability to down-regulate protein kinase C as well as to phosphorylate p80 and to transmodulate the binding of epidermal growth factor, two well established effects of protein kinase C in Swiss 3T3 fibroblasts. This emphasizes the complexity, from a functional point of view, of protein kinase C activation "in vivo."