The effectiveness of trilostane and azastene as inhibitors of adrenal steroidogenesis was compared by in-vitro and in-vivo methods. A radioimmunoassay was developed for the measurement of cortisol in ovine plasma, incubation medium and tissue extract using a specific antiserum raised against cortisol 21-acetate,3-carboxymethyloxime:bovine serum albumin. Trilostane (20 μmol/l) decreased cortisol synthesis and release both in unstimulated and in ACTH-stimulated adrenal tissues in vitro. The same concentration of azastene had a lesser effect on unstimulated adrenals and was completely ineffective in blocking the stimulatory action of ACTH. In vivo, trilostane suppressed adrenal steroidogenesis in pregnant and cyclic ewes but the suppression in pregnant ewes was over a longer period, and after lower doses. It is concluded that trilostane had an inhibitory effect on ovine adrenal steroidogenesis both in vitro and in vivo.