It has been shown previously that the muscarinic cholinergic system in the nucleus raphe magnus (NRM) is involved in the modulation of nociception. In this study, we examined the direct actions of muscarine on the NRM neurons in a slice preparation. Muscarine (1–30 microM) produced a dose-dependent hyperpolarization in a subpopulation of the NRM cells that contain 5-hydroxytryptamine (5-HT). In voltage clamp, the muscarine-induced outward current reversed polarity at the potassium equilibrium potential and was characterized by strong inward rectification. The reversal potential was dependent on external potassium concentration, suggesting that the hyperpolarization induced by muscarine was mediated through an increase in an inwardly rectifying potassium conductance. 5-HT also hyperpolarized these cells by increasing the same inwardly rectifying potassium conductance. The concentration-response curve for muscarine (EC50 = 2.7 microM) was shifted in a parallel manner to the right by increasing concentrations of pirenzepine (300 nM to 3 microM) and methoctramine (50–200 nM). Schild analysis revealed that the equilibrium dissociation constant (Kd) was 230 nM for pirenzepine and was estimated to be less than 30 nM for methoctramine. These results indicate that the muscarinic receptor mediating the muscarine activation of the potassium conductance in these cells is of the M2 subtype. The present results suggest an inhibitory cholinergic postsynaptic modulation on the activity of a subpopulation of serotonergic neurons that are involved in antinociceptive function in the NRM.