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Rats were initially trained to discriminate LSD from saline in two-lever operant chambers by reinforcing responses only on one lever following intraperitoneal injections of 80 mug/kg of LSD and only on the other lever following saline injections. Choice responding during extinction periods (no water reinforcement for either response) indicated a high level of discriminability (95% correct) following either LSD or saline. A dose-response curve for LSD, obtained by tests for lever choice after injections of 10, 20, 30, 40, 50, and 60 mug/kg, indicated that 10 mug/kg produced only 30% responding on the LSD lever. This percentage was increased (to 83%) by reinforcing responding on the LSD lever following injections of 10 mug/kg. Subsequent tests indicated that doses of 5.0 and 2.5 mug/kg produced a majority of responses on the LSD lever. Since at these low doses LSD has few measurable biochemical or behavioral effects, we hypothesized that the discriminable cue of LSD is related to direct stimulation of central serotonergic receptors.