It is well known that electrical stimulation of an area (subthalamic locomotor region, STLR) of the posterior hypothalamus evokes locomotion as well as increases in cardiorespiratory activity. Uncertainty exists over whether these responses are due to stimulation of STLR neurons or to activation of fibers of passage originating outside this area. The purpose of the present study was to determine if stimulation of STLR neurons alone would elicit the cardiorespiratory and locomotor responses. Neurons were stimulated by microinjections of gamma-aminobutyric acid (GABA) antagonists (picrotoxin and bicuculline) into the posterior hypothalamus of anesthetized cats. Both picrotoxin and bicuculline produced increases in arterial pressure, heart rate and minute ventilation which were accompanied by locomotor movements of the limbs. Increases in arterial pressure, heart rate and phrenic nerve activity were also caused by picrotoxin microinjections in paralyzed, ventilated cats. Microinjections of GABA or a GABA agonist (muscimol) reversed all of these responses. In contrast, microinjection of GABA or muscimol into the STLR without a prior antagonist injection had only small, transient effects upon cardiorespiratory activity. However, microinjection of muscimol prevented the responses to a subsequent injection of bicuculline. These results indicate that: (1) stimulation of cell bodies alone in the subthalamic locomotor region of the hypothalamus produces all the cardiorespiratory and locomotor responses evoked by electrical stimulation; (2) the responses evoked by picrotoxin and bicuculline are due to an interaction with GABA receptors and (3) a GABAergic mechanism exerts a tonic depressive influence over the cardiorespiratory and locomotor systems by an action in the posterior hypothalamus.