Pigeons were trained to discriminate 5 mg/kg pentobarbital from saline under several multiple fixed-ratio fixed-interval schedules of food presentation. The following schedules were studied: multiple fixed-ratio 40 fixed-interval 18 s (mult FR40 FI18), mult FR10 FI18 s, mult FR10 FI180 s and mult FR90 FI10 s. After responding stabilized under each multiple schedule, generalization curves were determined for pentobarbital, amobarbital, diazepam, phencyclidine andd-amphetamine. Pentobarbital generated dose-dependent increases in responding under all schedule components; however, there was more responding on the drug key after low doses of pentobarbital under FI components than under FR components, except for the FR90 component of the mult FR90 FI10 schedule. This tendency for more responding on the drug key after low doses of pentobarbital under FI components than under FR components generally was observed for low doses of all of the drugs. Examination of data from individual subjects revealed that there was a greater tendency for birds to distribute responding on both keys (mixed responding) under FI components than under FR components, where responding after each dose was confined largely to one of the two response keys. Analysis of local rates of responding within the FI component of the schedules showed that responding under the FI components developed the typical FI scallop at all Fl-component durations. These data suggest that FI schedules with values between 10 and 180 s generate similar dose-effect curves with higher rates of responding on the drug key after low doses of drugs than under FR schedules with low response requirements; however, under schedules with higher FR requirements, the dose-effect curves for some drugs begin to look more like those under FI schedules.