Drugs that influence the activity of central serotonergic neurons by activating a 5-hydroxytryptamine subtype of receptor (5-HT1A) alter mood and perception. Previously, we demonstrated with whole-cell recordings from acutely isolated 5-HT-containing dorsal raphe (DR) neurons from the adult rat that 5-HT inhibited Ca++ current and activated K+ current in DR neurons. We now show that D-lysergic acid diethylamide (LSD) mimics the actions of 5-HT; it dramatically suppresses Ca++ current in a dose-dependent manner and activates an inwardly rectifying K+ conductance. Spiperone (0.2 microM), a 5-HT1A/5-HT2 antagonist, blocks the effect of both LSD and 5-HT. The nonhallucinogenic structural analog 2-bromo-LSD (2-Bol) at 10 microM has no effect on either Ca++ or K+ current by itself, but it competitively antagonizes both effects of LSD. Inhibition of 5-HT release resulting from 5-HT1A receptor activation may play an integral role in the hallucinogenic actions of LSD by reducing competition between 5-HT and LSD for the postsynaptic 5-HT receptors.