The effects of 12 weeks treatment with probucol on plasma lipoprotein subfraction levels and on LPL and HTGL activities were investigated. Plasma VLDL-C, VLDL-TG, VLDL-apo B levels were not changed. Probucol significantly reduced plasma IDL-C and IDL-apo B levels by 26.7% and 23.8%, respectively. Plasma cholesterol and apo B levels of large light LDL (LDL1) were decreased significantly by 27.8% and 23.2% by probucol treatment. Plasma cholesterol and apo B levels of small heavy LDL (LDL2) remained unchanged. Probucol markedly reduced plasma HDL2 levels. The reduction rates of plasma TC, TG and apo A-I levels of HDL2 were 43.0%, 43.6% and 47.0%. Probucol significantly decreased HDL3-C and HDL3-apo A-I levels by 18.0% and 19.2%. LPL activities in the post-heparin plasma were decreased significantly from 2.53 +/- 0.71 mumol free fatty acids (FFA)/ml/h to 1.71 +/- 0.71 mumol FFA/ml/h by probucol while HTGL activities remained unchanged. We conclude that probucol suppresses LPL activity and decreases plasma IDL, LDL1 and HDL2 levels due to disturbances of VLDL conversion to LDL1 via IDL and of HDL3 conversion to HDL2.