Although ketamine's primary site of action appears to be the phencyclidine receptor on the N-methyl-D-aspartate (NMDA) receptor complex, additional activity on opiate and quisqualate receptors is suggested. Some phencyclidines have been shown to interact with muscarinic receptors, but this has not been determined for ketamine. We studied the interaction between ketamine and the m1 muscarinic receptor, the most prominent subtype in cortex and hippocampus. Receptors were expressed recombinantly in Xenopus oocytes, and intracellular Ca2+ release in response to the agonist acetyl-beta-methylcholine (MCh, 10(-6)M) was assessed by measuring charge movement through Ca(2+)-activated Cl- channels. Average responses to MCh were 4.1 +/- 0.7 microC. Ketamine inhibited responses to MCh, with complete inhibition at approximately 200 microM ketamine. The IC50 was 5.7 microM, (1.56 micrograms/mL), well within the clinically relevant concentration range. To demonstrate that intracellular signaling pathways and the Ca2+ activated Cl- channel were not affected by ketamine, we tested the effect of ketamine (365 microM) on currents induced by angiotensin II (10(-6) M) in oocytes expressing the AT1A angiotensin receptor. No inhibitory effect was noted. In summary, ketamine profoundly inhibits muscarinic signaling. This effect might explain some of the anticholinergic clinical effects of ketamine, both central (effects on memory and consciousness) and peripheral (prominent sympathetic tone, bronchodilation, mydriasis).