The main area of interest of this dissertation was to establish the mechanism of action of the hypolipidaemic drug, fenofibrate. Rats were treated with fenofibrate in the diet to provide intakes of 13, 60 or 200mg/kg/body weight/day for periods varying from 24 hours up to 18 months. Clofibrate (400mg/kg/body weight/day) served as a positive control. Groups of animals were killed at intervals during the course of the study. The aim was to establish the time course and dose response relationship of changes observed in the liver and plasma and to relate these to the morphological effects seen. The results indicated that the accumulation of fat within the liver is subsequently linked to an induction of enzymes responsible for the metabolism of fat in both the peroxisomal and microsomal fractions. The results obtained did show a number of drug-related effects, principally on the liver, which were markedly time and dose-dependent. There was a short-lived increase in the mitotic rate. Within 24 hours of offering the rats diets containing fenofibrate or clofibrate there were pronounced changes in the distribution of fat in the liver with a general loss from the centrilobular zone and an apparent accumulation in the periportal region. Electron microscopy showed proliferation of peroxisomes. Biochemically there was significant induction of the peroxisomal enzymes responsible for cyanide-insensitive palmitoyl CoA oxidation. Total catalase was induced at all time points beyond 14 days; however, catalase activity associated with particles sedimentable on centrifugation for 15 minutes of 10,000 xg was characteristically reduced. There was induction of the specific P450 enzyme, laurate hydroxylase, capable of metabolising w and w-1 oxidation products. There was a dose-dependent depression of glucose 6-phosphatase estimated both at the morphological and biochemical level. Other changes were slower to develop. After 14 days of treatment PAS positive diastase resistant granules (enlarged lysosomes) were noted near bile canaliculi in the liver and kidney proximal tubules. Lipofuscin deposits in liver were first visible after eight weeks of treatment. The lowest dose of fenofibrate (13mg/kg/body weight/day) produced no lipofuscin deposits. Biochemically, lysomal marker enzymes were significantly induced in all rats treated with 60 or 200mg/kg/body weight/ day of fenofibrate or 400 mg/kg/body weight/day clofibrate but not in rats treated with 13 mg/kg/day fenofibrate. A number of changes were also seen in serum parameters namely serum alanine transaminase and the thyroid hormones thyroxine arid triiodothyroxine. Morphological changes were reported in the thyroid. The results indicated that the accumulation of fat within the liver is subsequently linked to an induction of organelles and hence enzymes responsible for the metabolism of fat. Furthermore, the disproportionate increase of catalase suggests that the increase in the hydrogen peroxide (the end product of peroxisomal B-oxidation) may cause damage to membrane lipids and an eventual neoplasia. Investigations were carried out to establish whether the effects seen were only sex-linked. An investigation was carried out using di-2-ethylhexyl phthalate at doses of 50, 200 and 1000 mg/kg body weight/ day for a period of 3 days to 9 months in both male and female wistar rats. The effect of structure-activity was also investigated. The effects of di-G-ethylhexyl) phthalate were compared to those of the two straight-chain compounds di-n-hexyl phthalate and di-n-octyl phthalate. The investigation was carried out using the maximal tolerated dose of 2000mg/kg body weight/day for a period ranging from 3 days to 21 days. The straight-chain phthalates did not cause peroxisome proliferation and the fat seen to accumulate was centrilobular. The fat accumulation was associated with centrilobular necrosis. To test the hypothesis as to whether fat accumulation was associated with peroxisome proliferation chlorpromazine, a necroleptic drug, was investigated. This was shown to cause centrilobular fat accumulation without necrosis. Thus male wistar albino rats were given 25mg/kg/day chlorpromazine for periods ranging from 7 days to 90 days. The results obtained demonstrated that the large fat droplets seen to accumulate centrilobularly did not necessarily lead to peroxisome proliferation. The purpose of this thesis is to attempt to establish the mechanism by which the hypolipidaemic agents cause hepatocarcinoma.