As time increased between drug administration and the start of experimental sessions, effects of drugs on food-maintained responding in rhesus monkeys increased to a maximum and then decreased. d-Amphetamine, ethanol, and alpha-l-acetylmethadol (LAAM) generally decreased high response rates in one component of a chain schedule, while very low response rates in another component were increased reliably only by ethanol. The time of peak LAAM and ethanol concentrations in blood or plasma corresponded with or overlapped the time of maximal behavioral effect, while the time of maximal behavioral effect with d-amphetamine occurred somewhat prior to the time of peak plasma-amphetamine concentration. With d-amphetamine and perhaps with ethanol, effects on operant responding were greater after 30-min pretreatment intervals than after six-hr pretreatment intervals despite higher plasma or blood concentrations at six hours than at 30 min.