Acute and chronic effects of mephenytoin (30-360 mg/kg) were examined in pigeons responding under a multiple fixed-ratio 50 fixed-interval 90-sec schedule of food delivery. The highest dose administered acutely (240 mg/kg) produced substantial reductions in rate of responding under both components of the multiple schedule; the effects of other doses were small and inconsistent. With chronic exposure tolerance appeared to develop to the rate-decreasing effects of the drug.