Some effects of delta-amphetamine and pentobarbital on performance under a long fixed-interval schedule.

The effects of delta-amphetamine and pentobarbital were studied on performance during 3-hr sessions under fixed-interval 60-min schedules of food presentation. Low doses of delta-amphetamine increased rates of responding and higher doses decreased rates of responding, both during the entire 3-hr session and during each of the individual fixed intervals. Pentobarbital produced little effect on rates of responding averaged over the 3-r session, but it decreased rates during the first fixed interval and increased them during the second and third fixed intervals. The effects of delta-amphetamine were shown to be dependent on the control rate of responding, as has been shown with shorter fixed-interval values. Analysis of delta-amphetamine effects in terms of the point at which the probability of responding is greater than zero was not descriptive of overall fixed-interval performance.