The oscillatory current response to acetylcholine (ACh) in Xenopus laevis oocytes, mediated by endogenous muscarinic ACh receptors, is known to be mildly desensitized by repetitive ACh applications. Pretreatment of oocytes with staurosporine (an inhibitor of protein kinases) was found not only to abolish this desensitization but also to positively and progressively potentiate oscillatory ACh responses. This sensitization by staurosporine was suppressed by 12-O-tetradecanoylphorbol 13-acetate (an activator of protein kinase C). In staurosporine-untreated (control) oocytes, intracellularly injected calcineurin (an isozyme of Ca2+/calmodulin-dependent protein phosphatase 2B) or Ca2+ enhanced oscillatory ACh responses, while trifluoperazine (a calmodulin inhibitor) suppressed the ACh responses but did not affect oscillatory responses to intracellularly injected inositol 1,4,5-trisphosphate. These results suggest that, as far as short-term changes in receptor responsiveness are concerned, endogenous muscarinic ACh receptors in Xenopus oocytes are desensitized by phosphorylation by protein kinase C and sensitized by dephosphorylation by Ca2+/calmodulin-dependent protein phosphatase 2B.