Correlations between dyslipidemia and high blood pressure have been shown. It is unclear whether there is a causal relationship but there are indications that blood lipids might have a direct effect on mechanisms regulating the vascular tone. As secretory products of endothelial cells (EC) have been suggested to be involved in blood pressure regulation, we studied the influence of low density lipoprotein subfractions on thromboxane (T) synthesis in human umbilical vein endothelial cells (HUVEC). LDL can be separated into at least three major subfractions. In the present study, subfractions LDL1 (d = 1.030-1.033 g/ml), LDL 2 (d = 1.033-1.040 g/ml), and LDL 3 (d = 1.040-1.045 g/ml) were obtained by density gradient ultracentrifugation. T was measured by a radioimmuno-assay. LDL subfractions induced a dose dependent (10-100 micrograms/ml) increase in T synthesis. Control T concentration was 96 +/- 12 pg/ml. LDL 3 (205 +/- 17 pg/ml, p < 0.01) and LDL 2 (194 +/- 13 pg/ml, p < 0.05) caused a significantly higher T concentration compared to the LDL 1 subfraction (152 +/- 11 pg/ml). These results indicate that LDL 2 and 3- the subfractions with the higher density-might have a more pronounced effect on T synthesis by EC than the other LDL subfractions.