The concentrations of ACTH and testosterone (T) in the plasma from 12 normal women (aged 18–35 years) were simultaneously determined at various times after im injection of 1 mg corticotrophin. The increase in the plasma level of total immunoreactive ACTH was maximal 0.5 h after the injection (mean value 1123 as compared to 36 pg/ml in the basal state); at 48 h, the ACTH level was still 2.4-fold that measured in the basal state. The administration of corticotrophin induced a broad increase in the mean level of plasma T which was highly significant (P < 0.001) at 4, 6, 8, 16 and 24 h after the injection. The maximum T response was detected at 16 h (mean value 477 as compared to 338 pg/ml in the basal state). The results indicate that plasma T is susceptible to stimulation by a pharmacological dose of ACTH in normal women. This effect should be taken into account in evaluating the indication of ACTH as a therapeutic agent.