18 patients were studied with primary hypercholesterolemia (type II according to Fredricksen) and fasting cholesterol level during low fat diet above 250 mg/dl. Blood samples were taken: I--before lovastatin administration on diet, II--after 4 weeks administration of 20 mg, III--after 4 weeks of 80 mg and IV--4 weeks after the drug discontinued. The following tests were performed: platelets aggregation induced by ADP and adrenaline, PF3 and PF4, platelet MDA, serum triglycerides, total cholesterol, HDL-cholesterol and LDL-cholesterol. Before treatment patients showed significant platelets hyperaggregation and MDA concentration comparing to the control group. During the drug administration significant lowering of platelets MDA concentration, was observed small but significant HDL cholesterol level increase which correlated with platelet MDA concentration. Availability of PF3 and release of PF4 did not change during study.