The effects of 16 drugs were studied in rats responding under fixed-ratio (FR 30) and fixed-interval (FI 2 minute) schedules of food presentation. The drugs tested included lysergic acid diethylamide (LSD), dimethyltryptamine, mescaline, d-amphetamine and 12 methoxylated amphetamines. All of the drugs decreased the average rates of responding under both schedules, but their potencies varied widely. For example, with LSD, the most potent drug tested, doses of 0.1 to 0.3 mg/kg were sufficient to reduce responding while with dimethyltryptamine and mescaline, doses of 10 to 30 mg/kg were required to clearly reduce responding. For the 12 drugs which are known to produce hallucinogenic effects, their potencies in reducing responding were positively correlated with their reported potencies in producing these subjective effects in humans. Although all of the drugs decreased the average rates of responding, alterations in the patterns of responding under the FR and FI schedules varied among the drugs. Analysis of responding under the FI schedule indicated that d-amphetamine, m-methoxyamphetamine, p-methoxyamphetamine, LSD and 3,4-methylenedioxyamphetamine generally increased the low rates of responding occurring at the beginning of each interval and decreased the high rates of responding occurring later in each interval (rate-dependent effects). The other drugs generally decreased responding throughout the interval. These results are discussed in terms of the known neurochemical effects of these drugs.