1. Reproduction tests were carried out in rats receiving diets containing 10 p. 100 fresh or heated linseed oil (L) or 20 p. 100 fresh or heated rapeseed (Cz) or canbra (Cb) oils. These oils were heated either at 275 degrees C (12 hours, under nitrogen), or at 200 degrees C (100 hours or 60 X 30 minutes, exposed to the air). With L-275, we observed in the young rats a very important perinatal mortality and an hepatic hypertrophy with steatosis. With L-200, the mortality was lower, but the body growth of surviving young rats was slower and their liver were enlarged. With Cz-275 and Cb-275, the results were qualitatively identical, but less important that with L-275. With Cz-200 and Cb-200, a slowing down of the body growth and an enlargement of the liver were observed. With all the heated oils, cyclic monomers were detected in liver lipids of young rats before weaning. 2. The detoxification mechanisms of components formed during heating of the oils were studied in the weaned rat. We noted that a diet containing 10 p. 100 heated linseed oil markedly increased the urinary excretion of hippuric acid and mainly of glucuronic acid as compared to the results obtained with a 10 p. 100 fresh oil diet. Kinetics of excretion of conjugated glucuronic acid shows that there are two phases: a phase of fast increase of excretion (appearance of the detoxification system), then a phase where this increase was much lower. The authors draw some practical conclusions from their whole work.