In an attempt to determine which conformational parameters are important for the biological activity of ovine corticotropin-releasing factor (oCRF), we have synthesized in significant amounts (50-200 mg) and characterized chemically, structurally (CD), and biologically, oCRF analogues with substitution of each amino acid by its corresponding D-isomer. Out of 37 of these analogues, three were found to be equipotent to, or twice as potent as, oCRF, 13 had potencies in the range from 10 to 60%, 17 had potencies ranging from 1 to 10%, and the four others had potencies less than 0.5%. None of the analogues antagonized oCRF-induced release of ACTH in vitro at concentrations > or = 1000 oCRF. Since antagonists to CRF action can be generated by deletion of the first 8-14 residues, a series of CRF antagonists which exhibit significantly higher in vitro and in vivo biological potency than [Met18,Lys23,Glu27,29,40,Ala32,41,-Leu33,3 6,38] h/rCRF, [alpha-helical-CRF9-41], is also described. [D-Phe12,Nle21,38,Arg36]h/rCRF, in particular, was found to be ca. 15 times more potent than alpha-helical-CRF9-41 in vitro. In the rat, however, this analogue was about as effective as alpha-helical-CRF9-41 in blocking CRF-induced decrease in mean arterial blood pressure and increase in heart rate. Its potency in blocking epinephrine release by CRF was not significantly different from that of alpha-helical-CRF9-41. In the adrenalectomized rat, [Lys36]alpha-helical-CRF(9-41) (1.7 mg/kg) blunted the effect of endogenous CRF over a 90-min period; by comparison, a similar dose of alpha-helical-CRF9-41 was effective for less than 1 h.