Pigeons were exposed to fixed-interval schedules of food reinforcement with durations of 300 sec, 100 sec, or 40 sec. A range of doses of d-amphetamine was administered to each pigeon, and the resulting behavior was analyzed at several levels of detail. Average rates in different portions of the intervals predicted the magnitude of the drug's effect, but a finer analysis showed that average rates did not adequately characterize the behavior in some parts of the intervals. The probability of responding in different parts of an interval without drug was also a good predictor of the magnitude of the effect of d-amphetamine, and at the same time was more descriptive of the interval-to-interval performance. Analyses of the control performance indicated that responding in individual intervals could be described as consisting of two parts: a very low, or zero, rate at the beginning of the interval followed by an abrupt transition to a slightly, but reliably, positively accelerated rate maintained until reinforcement.