The postprandial plasma lipoprotein pattern was studied in 10 normal and 10 hypertriglyceridaemic subjects after consumption of either a saturated or a polyunsaturated fat-rich meal. Plasma triglycerides increased in both groups 3 h after the meal, and this was followed after 5 h by a dramatic reduction in the normal subjects only; the reduction was less after the saturated fat meal than after the polyunsaturated fat meal. This plasma triglyceride pattern was a consequence of changes in the chylomicron and very-low-density lipoprotein (VLDL) fractions. No significant changes were found in high-density lipoprotein (HDL)- and low-density lipoprotein (LDL)-cholesterol, triglycerides or protein concentration. Plasma cholesterol and apolipoproteins (apo) A-I and B were not significantly altered. The VLDL-apo C-III/apo C-II ratio increased 3 h after the saturated fat-rich meal, but decreased after the polyunsaturated fat-rich meal in normals, but not in the patient group. The effect of these postprandial lipoproteins on platelet function was studied by incubating normal washed platelets with the lipoprotein and then determining aggregation and [14C]serotonin release. All chylomicron fractions decreased platelet activity, whereas postprandial VLDL increased platelet activity. Five hours after the meals, the effect of VLDL on platelet activation was reduced in normal subjects only. The effect of postprandial LDL and HDL on platelet function differed little from that of the fasting lipoproteins.(ABSTRACT TRUNCATED AT 250 WORDS)