An influence of thyroid status on the secretory activity of hypothalamic dopaminergic neurons was observed in adult rats and its involvement in the regulation of prolactin (PRL) secretion was examined. The secretory activity of the tuberoinfundibular dopaminergic (TIDA) neurons was evaluated by measurement of dopamine (DA) biosynthesis in the neurons and DA release into hypophysial portal blood. The accumulation of DA and PRL in the adenohypophysis as well as PRL concentration in plasma were also estimated, and the various parameters were studied in thyroidectomized (TX), sham TX, TX rats treated for 7 days with thyroxine (T4; 20 µg/kg body weight daily) as well as in intact rats treated similarly with T4. An enhanced secretory activity of the TIDA neurons was observed in TX compared to sham TX rats, as attested by an increased synthesis of DA in the neurons, a greater concentration of DA in hypophysial portal plasma as well as an augmented accumulation of DA in the adenohypophysis. In the same animals, PRL was reduced in the adenohypophysis and in plasma, reflecting a blunted secretion of PRL in severe hypothyroidism. Treatment of TX rats with T4 for 7 days abolished all effects observed in TX rats, DA synthesis in TIDA neurons of TX rats treated with T4 being even less than in neurons of sham TX animals. A similar treatment with T4 administered to intact rats did not affect the secretory activity of the TIDA neurons nor the secretion of PRL. The present results are supportive of a modulatory influence of thyroid status on the secretion of DA by the TIDA neurons and consequently on PRL secretion by the adenohypophysis in the adult rat, and they are suggestive of direct effects of thyroid hormones on the TIDA neurons