The haemodynamic effects of i.v. bolus injections of 100 and 200 micrograms ovine CRF and human CRF in man have been compared. Neither ovine CRF 100 micrograms nor human CRF 100 micrograms caused a significant change in blood pressure, although the pulse rate was increased in all the subjects tested. The mean maximum increase in pulse rate after human CRF was almost twice that after ovine CRF (21 vs 12 beats X min-1; p less than 0.05). After 200 micrograms ovine CRF in all subjects the diastolic blood pressure declined gradually from 77 mm Hg to a nadir of 67 mm Hg at 22 min (p less than 0.002). After 200 micrograms human CRF diastolic blood pressure fell from 78 mm Hg to a nadir of 61 mm Hg at 6 min (p less than 0.002); the fall after human CRF was significantly greater than after ovine CRF (p less than 0.05). After 200 micrograms ovine CRF there was a slight increase in pulse rate lasting for 6 min, and after 200 micrograms human CRF there was a marked (reflex) tachycardia for 30 min. Only after the highest dose of human CRF did a slight increase in systolic blood pressure occur. The haemodynamic effects of both doses of human CRF were accompanied by significant increases in plasma noradrenaline concentrations, which were significantly greater after the higher dose.