Chronic toxicity studies of fenofibrate were carried out in rats (3 months), dogs (7 et 24 months) and Rhesus monkeys (12 months). The results in the last named species (78 animals) were of particular interest, since the treated monkeys had normal size liver without histological abnormalities. Electron microscopy showed no increase in the number of hepatic peroxisomes. Long-term toxicity studies in rats failed to show any increase in the incidence of altered hepatocytes or of neoplastic tumours in treated animals. However, a few peroxisomes were found in animals receiving the highest doses of fenofibrate. In reproduction studies there was no evidence of teratogenic effects in rats with doses 45 times higher than the human dose, nor in rabbits with doses of 300 mg/kg/day. In mutagenicity studies fenofibrate proved unable to bind with DNA and could not, therefore, have any effect on genes. The side-effects encountered in clinical practice (e.g. digestive disorders, sexual fatigue, myalgia, alopecia) were rare and obliged to discontinue treatment in very few cases. Long-term clinical trials failed to demonstrate any fenofibrate-induced pathology, such as malignant or benign tumours, or biliary or urinary lithiasis. Serum transaminases were increased in 10 to 20% of the patients, but the rise was transient and never reached pathological levels. Electron microscope study of liver biopsies from patients treated with fenofibrate showed no proliferation of peroxisomes.