Polyunsaturated fatty acids play an important yet poorly understood role in pregnancy complications. We are interested in the effects of aspirin therapy on the metabolism of these compounds. To determine the effects of low-dose aspirin on plasma levels of polyunsaturated fatty acid precursors, we assayed linoleic, linolenic, arachidonic, eicosapentaenoic, and docosahexaenoic acids using high-performance liquid chromatography. Seventeen pregnant women being treated with aspirin therapy (81 mg/day) were evaluated between 9 and 37 weeks' gestation. Blood was drawn before initiation of aspirin therapy and after 3 or 4 days, and again after 3 or 4 weeks of therapy. We found no significant change in the plasma levels of fatty acids during aspirin therapy at either 3 or 4 days or 3 or 4 weeks compared with baseline. In this group of women with preexisting disease, low-dose aspirin does not appear to change the plasma levels of polyunsaturated fatty acid precursors of eicosanoids. Data on pregnant women without hypertensive disorders is needed to help in understanding the role and physiology of these important compounds.