The postoperative regulation of food and water intake was studied in rats with aspiration lesions to either the medial frontal or orbital frontal projection fields of thalamic nucleus medialis dorsalis (prefrontal cortex). These projection fields proved functionally dissociable in that orbital frontal lesions impaired immediate postoperative regulation of food and water intake for up to 2 wk., while medial frontal lesions produced finickiness. Neither lesion affected response to cellular dehydration or recovery from extended deprivation. These data are consistent with data from rhesus monkeys with prefrontal lesions and differ from animals with lateral hypothalamic lesions.