The effects of bilateral frontal cortical lesions on spontaneous diurnal rotation and locomotor activity were studied in female Sprague-Dawley rats and determined to be a function of preoperative directional bias. Such lesions decreased rotation and activity in rats with right-sided biases and increased rotation and activity in rats with left-sided biases. It is suggested that frontal cortex normally modulates a dopaminergic nigrostriatal asymmetry and that an asymmetry and that an asymmetry in corticostriate function, possibly involving glutamate, underlies the observed effects.