Demonstration of guanine-nucleotide-dependent neurotransmitter stimulation of phosphoinositide breakdown in brain membranes has generally required the presence of the detergent, deoxycholate (DOC), in the assay medium. In the present study, by using mouse brain cortical membranes labelled with [3H]inositol in the presence of CMP through the reverse PtdIns synthase reaction, we have been able to show guanosine 5'-[γ-thio]triphosphate (GTP[S])-dependent carbachol (CCh) stimulation of the formation of [3H]inositol phosphates in the absence of DOC and have studied how the detergent affects the response. The results of our study indicate that DOC affects the muscarinic receptor-G-protein-phosphoinositidase C (PIC) transduction system in several ways. First, it enhances agonist-induced PIC activity towards [3H]PtdInsP and [3H]PtdInsP, and, secondly, it decreases the potency for GTP[S] stimulation of PIC, thus enhancing the agonist-induced leftward shift of the dose-response curve for GTP[S]. Additionally, DOC appears to increase the activity of the enzymes of the phosphoinositide cycle, PtdIns 4-kinase,Ins(1,4,5)P3 5-phosphatase and Ins (1,4)P2 1-phosphatase thus altering the proportion of phosphoinositide substrates and inositol phosphate products. These observations advise caution in drawing conclusions about PIC substrate specificity and the potency of both guanine nucleotides and agonists from experiments performed in membranes in the presence of DOC or related bile salts.