BR-931 [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio-(N-beta-hydroxyethyl)-acetamide], a new hypolipidemic agent of low toxicity, was evaluated in several tests of lipolysis and hyperlipidemia in rats, and in the cholesterol-induced atherosclerosis in rabbits. Significant hypolipidemic activity was observed in rats with doses of the agent at 12.5--50 mg/kg. In the Triton-induced hyperlipidemia, 50 mg BR-931 per kg was equieffective as 200 mg of clofibrate (CPIB) per kg. In contrast with CPIB, BR-931 exerted a powerful antilipolytic activity against epinephrine, ACTH, nicotine and cold exposure. BR-931 was particularly effective in diet-induced hyperlipidemias. Ethanol lipemia was totally prevented by the agent at 100 mg/kg. With Nath's diet, doses as low as 25 mg/kg significantly reduced hypercholesterolemia and hypertriglyceridemia. In these last two tests, the distribution of lipoprotein cholesterol was also determined. CPIB did not affect HDL cholesterol levels that had been decreased by the diets; in contrast, BR-931, already at doses of 50 mg/kg, brought the HDL/total cholesterol ratio back toward normal. A significant HDL cholesterol increase, together with some reduction of atheromatosis, was also observed in cholesterol-fed rabbits. BR-931, a potent inducer of liver peroxisones and of mitochondrial carmitine acetyltransferase, appears to be a hypolipidemic agent of high efficacy and low toxicity for the clinical treatment of hyperlipidemias and atherosclerosis.