This study was undertaken to determine the plasma and erythrocyte profiles of metabolically important nonesterified polyunsaturated fatty acids (PUFA), that is, free fatty acids, in each trimester of pregnancy and in labor. Blood was drawn from patients in the first, second, and third trimester and in labor. Nonesterified polyunsaturated fatty acids were extracted from erythrocytes with chloroform and methanol. The PUFAs from erythrocytes and plasma were then measured in their methylester form using high performance liquid chromatography (HPLC). Plasma levels of all PUFAs were similar in each trimester of pregnancy but levels of linoleic and linolenic acids were higher in laboring patients. Plasma levels of linoleic and arachidonic acid in the n-6 pathway (range 40 to 162 mg/L plasma) were higher than linolenic, eicosapentaenoic, and docosahexaenoic acids measured in the n-3 pathway (range 2.1 to 12.8 mg/L plasma). PUFA levels in erythrocytes were generally higher in the second trimester (range 2.6 to 79.7 mg/100-microL spun erythrocytes). In these erythrocytes, docosahexaenoic acid in the n-3 pathway and linoleic and arachidonic acids in the n-6 pathways were present in the highest amounts. Polyunsaturated fatty acids appear to be absorbed and mobilized in increasing amounts in plasma and erythrocytes with advancing gestational age and labor. This activity appears to be most pronounced in the second trimester. Further investigations into PUFA metabolism and the mechanisms which govern it could lead to a better understanding of the role of these important substances in normal and abnormal pregnancies as well as in the initiation of labor.