The subacute toxic effects of cyclopiazonic acid (CPA; given orally) were characterized in the dog (CPA was purified from cultures of Aspergillus flavus). Four groups of dogs were given CPA in gelatin capsules for 90 days at the following dosage levels: 0.05, 0.25, 0.5, and 1.0 mg/kg of body weight; a 5th group was used as controls. All dogs administered the 0.5 and 1.0 mg of CPA/kg dosages and 1 dog given the 0.25 mg of CPA/kg dosage died or were humanely killed before the scheduled termination of the study. Clinical signs of intoxication appeared 2 to 44 days after dosing was started and consisted of anorexia and, in 1 to 2 days, vomiting, diarrhea, pyrexia, dehydration, weight loss, and CNS depression. Grossly, the entire alimentary tract had diffuse hyperemia with focal areas of hemorrhage and ulceration. Other lesions were renal infarcts, necrotizing epididymitis, and ulcerative dermatitis. Microscopic lesions included ulceration, necrosis, vasculitis, lymphoid necrosis, karyomegaly in several organs, and decreased mitotic activity in intestinal crypt epithelium. Ulcerative and necrotic lesions were usually associated with vascular lesions. Clinical pathologic changes were leukocytosis, neutrophilia, lymphopenia, monocytosis, and increased serum alkaline phosphatase activity.