The incertohypothalamic dopaminergic (DA) neuronal system has been divided into a rostral component of neurons originating in the rostral periventricular nucleus and projecting to the preopticosuprachiasmatic and medial preoptic nuclei and a caudal component originating in the medial zona incerta and projecting to the dorsomedial and anterior hypothalamic nuclei. The purpose of the present study was to determine if the activity of these intrahypothalmic DA neurons is regulated by DA receptor-mediated mechanisms, as are those in the major ascending nigrostriatal and mesolimbic neurons, or if they resemble another group of intrahypothalamic DA neurons, those that comprise the tuberoinfundibular system, which are not responsive to the acute actions of DA agonists or antagonists. The rate of DA turnover (decline after alpha-methyltyrosine) in micropunched regions of the striatum (ST), nucleus accumbens (NA) and hypothalamic regions which contain cell bodies or terminals of incertohypothalamic DA neurons was increased after administration of a DA antagonist (haloperidol) and decreased after administration of a DA agonist (bromocriptine). gamma-Butyrolactone increased DA concentrations in the ST, NA and hypothalamic brain regions containing incertohypothalamic DA neurons, and this effect was blocked by the DA agonist apomorphine. In contrast, none of these treatments affected the concentration or rate of turnover of DA in the median eminence (terminal region of tuberoinfundibular neurons). Injections of either gamma-hydroxybutyric acid or baclofen into the substantia nigra/ventral tegmental region of the midbrain increased DA concentrations in the NA and/or ST but failed to alter DA concentrations in any hypothalamic region. These results suggest that the incertohypothalamic DA system is composed of neurons whose activity can be rapidly modulated by DA receptor-mediated mechanisms and thus resemble the DA neurons in the major ascending nigrostriatal and mesolimbic systems rather than the hypothalamic neurons which comprise the tuberoinfundibular DA system.