Intracellular recording from hippocampal CA1 pyramidal cells was used to characterize the pharmacological properties of muscarinic responses. Results obtained with the M1 antagonist pirenzepine and the M2 antagonist gallamine suggest that an M1 muscarinic receptor is involved in the muscarinic-induced membrane depolarization and blockade of the afterhyperpolarization (AHP). On the other hand, an M2 receptor may be involved in the cholinergic depression of the EPSP and the blockade of the potassium current termed the M-current. Pretreatment of hippocampi with pertussis toxin did not prevent any of the muscarinic responses suggesting that a pertussis toxin-sensitive G-protein is not involved. The M-current, in contrast to the other muscarinic actions, was unaffected by muscarinic agonists which are weak at increasing phosphoinositide (PI) turnover and actually blocked the action of full agonists. This finding suggests that stimulation of PI turnover may be involved in the blockade of the M-current. Although activation of protein kinase C with phorbol esters has little effect on the M-current, intracellular application of inositol trisphosphate did reduce the M-current. We were unable to establish any clear relationship between biochemical effector systems and the muscarinic receptor subtypes.