To evaluate the incidence of hyperprotactinemia, and to explore the possible altered dopaminergic effect on the secretion of prolactin and TSH in hypothyroidism, metoclopramide-dopamine receptor blocker was administered to 31 patients with hypothyroidism and 15 normal controls. Blood samples were drawn just b fore and 30 minutes after metoclopramide 10 mg intravenous bolus injection, for T4 TSH, and prolactin. Mean basal level of prolactin was much higher in patients than in controls. (30.1±23.5 ng/ml vs. 11.1±3.0 ng/ml) Twentyone out of the thirty-one patients(68%) had hyperprolactinemia defined as serum prolactin values above 20 ng/ml, and four patients(13%) grossly obvious galactorrhea. Increase in prolactin after metoclopramide administration was significantly lower in patients with hypothyroidism than in controls(p<0,01), which suggested that dopaminergic activity was reduced in hypothyroid patients. No difference was seen in TSH response to metoclopramide between patients and controls. Serum T4, levels were inversely correlated with the common logarithm of serum prolactin and TSH levels(prolactin; r=-0.45, P<0.01 TSH; r=-0.85, p<0.001). Increase in prolactin and TSH after metoclopramide had inverse relationships with their own basal levels. In conclusion it was considered that 1) serum prolaetin level was elevated in hypothyroidism, 2) the degree of hyperprolactinemia was inversely related to thyroid status, 3) dopaminergic inhibitory activity on prolactin was decreased in hypothyroid patients, and hyperprolactinemia in hypothyroidism might be due to decreased dopamine-rgic inbibition on prolactin secretion, and 4) dopaminergic activity on prolactin and TSH secretion was not parallel.