The effects on plasma ACTHlike activity and upon peripheral plasma 17- hydroxycorticosteroids (17-OHCS) of intravenous methopyrapone (Metopirone) were studied in normal dogs. Significant changes in the peripheral plasma 17-OHCS were noted during the 8-hr intravenous administration of methopyrapone in 6 dogs when compared with control animals. Plasma ACTH-like activity of the same animals, bio-assayed in the hypophysectomized isolated dog adrenal pouch preparation, was significantly increased in all 6 experiments after 8 hr of methopyrapone, in comparison with that present in the pre-methopyrapone plasma and with the ACTH-like activity of plasma from control animals identically infused but without the addition of methopyrapone. The evidence presented supports the view that plasma ACTH-like activity increases upon intravenous methopyrapone administration in the dog.