In an earlier report we identified specific modifications and substitutions of corticotropin releasing factor (CRF) that led to the discovery of antagonists with extended duration of action as compared to that of astressin {cyclo(30-33)[DPhe(12),Nle(21),Glu(30), Lys(33),Nle(38)]hCRF((12)(-)(41))}. These additional modifications included elongation of the peptide chain by three residues at the N-terminus, its acetylation, and the [CalphaMeLeu(27)] substitution to yield cyclo(30-33)[DPhe(12), Nle(21),CalphaMeLeu(27),Glu(30), Lys(33),Nle(38)]Ac-hCRF((9)(-)(41)), which was found to be longer acting than astressin (Rivier, J.; et al. J. Med. Chem. 1998, 41, 5012-5019). To further increase the efficiency (potency, duration of action, and bioavailability) of this family of antagonists, we introduced two or more CalphaMe-leucine residues at positions shown in earlier studies to be favorable (Hernandez, J.-F.; et al. J. Med. Chem. 1993, 36, 2860-2867). Whereas the introduction of CalphaMe-leucine residues at positions 27 and either 18 (11), 37 (17), or 40 (19) resulted in dramatic increases in duration of inhibitory action in the adrenalectomized (adx) rat after intravenous injection, the same substitution at positions 27 and either 15 (7, 8), 17 (9), 19 (12, 13), or 41 (20) led to short acting analogues. Other substitutions by CalphaMeLeu at positions 27 and either 10 (4), 13 (5), 14 (6), 21 (14), 24 (15), 36 (16), or 38 (18) yielded analogues with duration of action intermediate between those mentioned above. Cyclo(30-33)[DPhe(12), Nle(21), CalphaMeLeu(27),Glu(30),Lys(33),Nle(38), CalphaMeLeu(40)]Ac-hCRF((9)(-)(41)) (astressin B, 19) was one of the most efficacious analogues of this series (>4 h inhibition of ACTH secretion at 25 microgram/adx rat). It was found to be even longer acting via subcutaneous administration in either an aqueous (>24 h inhibition of ACTH secretion at 100 microgram/adx rat) or lipid milieu (DMSO/peanut oil, >24 h inhibition of ACTH secretion at 30 microgram/adx rat) than after intravenous administration (<12 h inhibition of ACTH secretion at 100 microgram/adx rat). We concluded that Calpha-methylation at some positions may favor a bioactive conformation while also preventing degradation and/or elimination, resulting in significant extension of duration of action.