Synthetic ovine corticotropin releasing factor (oCRF) and human CRF were given as an intravenous bolus to 6 healthy volunteers in 4 different dosages (oCRF: 25, 50, 100 and 200 µg) respectively, 2 different dosages (hCRF: 50 and 100 µg). ACTH and cortisol were measured over a 2-hour period after CRF injection. CRF immunoreactivity was measured with a newly developed homologous radioimmunoassay for both oCRF and hCRF. All CRF dosages tested led to a definite ACTH response compared to placebo control. No dose dependency of side effects was observed. Furthermore, there was no clear-cut dose response relationship between the injected CRF dosages and the maximal ACTH- and cortisol levels. However, when the area under the ACTH and cortisol-response curves after oCRF were compared a dose-response relationship emerged, which could not be demonstrated in the same way for the two hCRF dosages. Serum CRF immunoreactivity correlated to the injected dosage. The halftime of serum disappearance for oCRF was 18 min, and hCRF, 9 min. The maximal ACTH responses after injection of oCRF and hCRF were not significantly differen