We have studied the effects of the TRH related dipeptide histidyl-proline diketopiperazine [cyclo (His-Pro)] on basal and stimulated TSH and PRL secretion in normal volunteers, in patients with microprolactinomas and in patients with primary hypothyroidism. Cyclo (His-Pro), 400 micrograms intravenously did not alter basal TSH or PRL levels in normal males and females and was also without effect upon the elevated basal TSH and PRL levels in patients with primary hypothyroidism and microprolactinomas respectively. The same dose of cyclo (His-Pro) did not affect the TSH or PRL response to TRH (100 micrograms i.v.) in normal male volunteers. These data indicate that cyclo (His-Pro) does not affect TSH and PRL secretion in man at this dosage. It is also unlikely that this molecule will be of any therapeutic benefit in states of hyperprolactinaemia.