1 Many G protein-coupled receptors contain potential phosphorylation sites for protein kinase C (PKC), the exact role of which is poorly understood. In the present study, a mutant cholecystokinin A (CCK A ) receptor was generated in which the four consensus sites for PKC action were changed in an alanine. Both the wild-type (CCK A WT) and mutant (CCK A MT) receptor were stably expressed in Chinese hamster ovary (CHO) cells. 3 The dose-response curves for CCK-8-induced cyclic AMP accumulation and inositol 1,4,5-trisphosphate (Ins(1,4,5)P 3 ) formation were shifted to the left in CHO-CCK A MT cells. This leftward shift was mimicked by the potent inhibitor of protein kinase activity, staurosporine. However, the e ect of staurosporine was restricted to CHO-CCK A WT cells. This demonstrates that attenuation of CCK-8-induced activation of adenylyl cyclase and phospholipase C-b involves a staurosporine-sensitive kinase, which acts directly at the potential sites of PKC action on the CCK A receptor in CCK-8-stimulated CHO-CCK A WT cells. 4 The potent PKC activator, 12-O-tetradecanoylphorbol 13-acetate (TPA), evoked a rightward shift of the dose-response curve for CCK-8-induced cyclic AMP accumulation in CHO-CCK A WT cells but not CHO-CCK A MT cells. This is in agreement with the idea that PKC acts directly at the CCK A receptor to attenuate adenylyl cyclase activation. 5 In contrast, TPA evoked a rightward shift of the dose-response curve for CCK-8-induced Ins(1,4,5)P 3 formation in both cell lines. This demonstrates that high-level PKC activation inhibits CCK-8-induced Ins(1,4,5)P 3 formation also at a post-receptor site. 6 TPA inhibition of agonist-induced Ca 2+ mobilization was only partly reversed in CHO-CCK A MT cells. TPA also inhibited Ca 2+ mobilization in response to the G protein activator, Mas-7. These ®ndings are in agreement with the idea that partial reversal of agonist-induced Ca 2+ mobilization is due to the presence of an additional site of PKC inhibition downstream of the receptor and that the mutant receptor itself is not inhibited by the action of PKC.7 The data presented demonstrate that the predicted sites for PKC action on the CCK A receptor are the only sites involved in TPA-induced uncoupling of the receptor from its G proteins. In addition, the present study unveils a post-receptor site of PKC action, the physiological relevance of which may be that it provides a means for the cell to inhibit phospholipase C-b activation by receptors that are not phosphorylated by PKC.