Disappearance of immunoreactive ovine corticotrophin-releasing factor (IR-oCRF) from plasma after a single intravenous injection of ovine corticotrophin-releasing factor (oCRF) was studied in man in the morning and evening. Synthetic oCRF (80 micrograms) was injected intravenously to four normal male volunteers at 0900 h or at 2200 h. Blood samples were drawn before and 2, 5, 10, 15, 30, 45, 60, 90 and 120 min after the oCRF injection. Plasma IR-oCRF was measured by a specific radioimmunoassay for OCRF. Plasma concentrations of IR-oCRF after oCRF injections in the morning and in the evening did not differ significantly (P greater than 0.05). Disappearance of IR-oCRF was modelled with a two-exponent function by using a non-linear least squares computer program. The metabolic clearance rate, the apparent initial volume of distribution, the plasma half-life for the fast component and that for the slow component calculated from all eight tests in the morning and evening were 1.49 +/- 0.05 ml/min X kg, 44.4 +/- 1.7 ml/kg, 6.8 +/- 0.7 min and 46.2 +/- 2.3 min (mean +/- SEM), respectively. This relatively long half-life may be responsible for the prolonged biological effect of oCRF administered intravenously. There were no significant differences between parameters of IR-oCRF disappearance curves in the morning and those in the evening (P greater than 0.05).