A number of 11-deoxysteroid compounds were tested at low and differing dose levels for their lactation-maintaining effect in adrenalectomized rats, the compounds being implanted subcutaneously in tablet form. Reichstein's compound S acetate was active, but Reichstein's compound L, 21-acetoxy pregnenolone, Δ5-pregnenolone acetate, Δ16-progesterone, 17-vinyltestosterone, 17-hydroxy-21-acetoxy Δ1-allopregnene-3, 20-dione, 3-(β)-21-diacetoxy allopregnane-20-one and allopregnane-21-ol-3, 20-dione acetate were inactive. The ovaries of adrenalectomized lactating rats were found to exert a lactation-maintaining effect equivalent to that of about 3·1 mg progesterone daily. Progesterone showed a high threshold in its effect on lactation in adrenalectomized-ovariectomized rats. In adrenalectomized-ovariectomized lactating rats Reichstein's compound S acetate maintained lactation as effectively as deoxycorticosterone acetate (DCA) or cortisone. When used together in such rats DCA and cortisone acted synergistically. This synergism was not shown by cortisone and compound S acetate, but may have been shown to a lesser extent by DCA and compound S acetate. Lactational performance was not improved when the dose of DCA used daily with 1·1 mg cortisone was increased to above 0·43 mg daily, nor did the addition of 0·55 mg compound S acetate daily to this level of DCA and cortisone have a significant effect. Adrenalectomized-ovariectomized lactating rats given DCA and compound S acetate increased in body weight, whereas those given cortisone lost weight. There was a very high correlation between the mean food intakes and the mean litter-growth indices for treatment groups in all experiments.