Plasma concentrations of adrenocorticotropin (ACTH; corticotropin) were measured in 31 normal dogs (house pets) at rest. The mean concentration was 45.77 pg/ml of blood, with individual values ranging from 17 to 98 pg/ml of blood. The measurement of this hormone secreted by the anterior lobe of the pituitary gland (adenohypophysis) may aid in the diagnosis of spontaneous canine adrenal cortical disorders, both primary and secondary.