Serotonergic and non-serotonergic neurons in the nucleus raphe magnus were identified by conduction velocities and spontaneous discharge rates. Eight of 13 serotonergic neurons were depressed by lysergic acid diethylamide (LSD) administration (40-50 micrograms/kg, i.p. or i.v.), while the others were non-responsive. Among 38 non-serotonergic neurons, 5 were depressed, 4 were excited and 29 were...