Endogenous ACTH and cortisol was suppressed in horses with dexamethasone. Exogenous ACTH‐123I was infused for 3 h at a concentration to produce physiological levels of plasma cortisol. After reaching a plateau concentration of ACTH‐123I, cortisol concentration was constant throughout the remaining infusion period. Ultradian rhythms in cortisol secretion were abolished by the constant concentration of ACTH supporting the concept mat ACTH secretion is the primary regulator of cortisol rhythms. The half‐life of ACTH‐12’I in three horses was 4, 5 and 8 min following cessation of ACTH‐125I infusion.