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A multiple differential reinforcement of low rate 8 s discrete-trial, differential reinforcement of low rate 8 s free-operant procedure was used to investigate the effects of D-amphetamine on interval timing in pigeons. On the discrete trial differential reinforcement of low rate early responses terminated the trial; on the free-operant differential reinforcement of low rate early responses reset the reinforcement timer but had no other effects. Frequency distributions of log interevent times for responses following reinforcement on both the free-operant and discrete-trial components showed a single-peaked distribution. The interevent time distribution for responses preceded by unreinforced responses on the free-operant differential reinforcement of low rate showed two peaks. The pigeons were administered two doses of D-amphetamine for 20 successive sessions. The means of interevent time distributions did not change under drug for either condition but widths of the distributions increased in both conditions and for both types of interevent times in the free-operant differential reinforcement of low rate. In the free-operant condition the mean interevent time for responses preceded by prior responses declined significantly with drug owing to a reduction in the proportion of responses in the shorter interevent time distribution. In effect, response bouts were shortened by drug administration. The rate-dependent hypothesis well described the effect of amphetamine on responses in the free-operant procedure. These results are discussed in the context of amphetamine's effects on the temporal structure of behavior.