The effects of the dopamine (DA) receptor antagonist metoclopramide on the plasma thyroid stimulating hormone (TSH) and prolactin (PRL) levels were studied in 8 patients with subclinical hypothyroidism (defined as absence of clinical signs of hypothyroidism with normal thyroid hormone levels, normal or slightly increased basal plasma TSH levels and increased and long-lasting TSH response to TRH) before and after l-thyroxine replacement therapy. Metoclopramide induced a significant (p < 0.01) TSH release in the subclinical hypothyroid patients. Two weeks after l-thyroxine replacement therapy (50 µg/day), the TSH response to metoclopramide was completely blunted in subclinical hypothyroidism. In these patients a significant (p < 0.01) inhibition of TSH response to intravenous thyrotropin-releasing hormone (TRH) was also observed after treatment with thyroid hormone. In analogy to the TSH behavior, plasma PRL secretion in response to metoclopramide and TRH administration was significantly (p < 0.05) inhibited in the subclinical hypothyroid patients after l-thyroxine replacement therapy