Food-deprived rats were trained to press a key which produced a food pellet for the first press after 3 min had elapsed (FI 3 min). Daily sessions consisted of 10 such intervals. Graded doses of LSD (0.04 - 1.28 mg/kg) and a d-amphetamine (0.5 - 2.0 mg/kg) were given 30 min before sessions. LSD produced a decrease in response rate at doses of 0.32 mg/kg and above, but did not disrupt the typical FI pattern of responding except at the highest dose (1.28 mg/kg). Amphetamine did not significantly alter the overall response rate, but caused a dose-related disruption of the FI response pattern, with previously low response rates increased more than higher rates, and occasional decreases in the previously highest rates. The experiment was repeated using the same rats responding on a multiple FIFR schedule. The presence of a 2000 Hz tone signalled FR periods; the tone was absent during FI periods. During the FR components, a pellet was produced after 30 responses had been emitted. The FI components were unchanged. LSD (0.08 - 0.32 mg/kg) again produced decreases in FI rate without altering the pattern, and amphetamine again altered the FI pattern without significantly changing overall rate.