Adult cats were monitored for their performance of a variety of motor functions before and after acute administration of phenytoin (5, 10 or 20 mg/kg) in a schedule in which each animal received all drug doses. The only significant loss in motor function was balance and coordination. Half the animals could not balance or walk along a narrow-edged beam after 20 mg/kg of phenytoin although their performance was not impaired at lower drug doses or on wider surfaces. There were no effects of phenytoin on the righting reflex, flexor reflex, muscle strength, the hopping response, the blind placing response or visually aided placing. The data suggest that phenytoin has a selective effect on higher order neuronal systems involved with balance and locomotion rather than simple reflex pathways.