The corticosteroidogenicity of ACTH-derived peptides was tested in a purified isolated rat adrenal cell system. The activity was related to the activity of standard ACTH (synthetic hACTH 1 - 3 9 ; potency fixed at 100). The peptides ACTH 1 1 - 1 9 NH 2 and ACTH 1 1 - 2 4 showed steroidogenicity at pharmacological doses (potencies: 0.00067 and 0.00032, respectively). None of the peptides tested inhibited or potentiated the ACTH-induced steroidogenesis at low doses (0.5-50 000 pg). The results suggest the presence of a second center within the ACTH molecule capable of inducing steroidogenesis.