After an oral fat load of 1 g/kg body weight in 10 obese females with hyperlipoproteinaemia type IV, serum triglycerides concentrations were maximal at 4 h with a slight decline at 6 h, whereas serum cholesterol concentrations rose slightly at 4 h and 6 h. After 6 h, concentrations of triglycerides and cholesterol were significantly increased in chylomicrons and very low-density lipoprotein (VLDL), whereas cholesterol concentrations were decreased in high-density lipoprotein 2 (HDL2) plus HDL3. After oral treatment with 450 mg gemfibrozil twice daily for 28 days, triglyceride concentrations were reduced in serum, chylomicrons, VLDL and low-density lipoprotein, and total cholesterol concentrations were reduced in serum, chylomicrons and VLDL, and increased in HDL2 plus HDL3. At 6 h after a fat load following 28 days' gemfibrozil treatment, triglyceride and cholesterol concentrations were reduced in serum, chylomicrons and VLDL when compared with pretreatment results. It is concluded that gemfibrozil is effective in lowering triglycerides and cholesterol, particularly in triglyceriderich particles, and raising the cholesterol content of HDL2 plus HDL3. After an oral fat load gemfibrozil inhibits the increase in serum cholesterol and partly prevents postprandial hypertriglyceridaemia.