Administration of exogenous thyrotropin-releasing hormone (TRH) in man leads to an increase in mean arterial blood pressure at 4 and 8 min after TRH. This increase in mean arterial blood pressure is preceded by an increase in plasma norepinephrine levels (maximum response of 274 ± 42 pg/ml vs. basal 158 ± 32, p < 0.01). There was a nonsignificant tendency for epinephrine levels to rise. There was no alteration in plasma renin activity (basal 1.2 ± 0.4 ng/ml/min vs. peak 1.4 ± 0.3) or in plasma aldosterone (basal 5.0 ± 0.6 ng/dl vs. peak 6.8 ± 0.7). We conclude that TRH causes an increase in blood levels of catecholamines in normal man through undetermined mechanisms. The blood pressure changes associated with TRH administration may be related to norepinephrine release. Our findings also lend additional support that acute increases in prolactin have no effect on the renin-angiotensin-aldosterone system i