The effects of drugs on rate of responding by rats under a multiple fixed-ratio fixed-interval schedule of food presentation were studied before and during chronic administration of l-alpha-acetylmethadol (LAAM). Small doses of methadone, phencyclidine, chlordiazepoxide and pentobarbital increased rates of responding under the fixed-interval component of the schedule. At higher doses, these drugs, as well as LAAM, morphine and haloperidol, decreased rates of responding under both schedule components. Chronic LAAM administration shifted the dose-effect curves for LAAM, morphine, methadone, and to some extent, perhaps pentobarbital, to the right, but did not shift the dose-effect curves to the right for chlordiazepoxide, phencyclidine and haloperidol. The rate-increasing effects of small doses of methadone, phencyclidine, pentobarbital and chlordiazepoxide were attenuated during chronic LAAM administration.