Background: To assess the relationship between the mothers'intake of n-3 long chain polyunsaturated fatty acids (LC PUFA) during pregnancy and their levels in plasma and tissue. Methods: 162 mothers were studied during labor. Three group、 were differentiated according to the n3 LC PUFA intake assessed by means of a dietetic interview: superior intake (SIG) (>0.721 g/day), medium intake (MIG) (from 0.382 to 0.721 g/day) and inferior intake (IIG) (<0.381 g/day). Fatty acids (FA) were studied by capillary chromatography in plasma and in erythrocyte pho­spholipids. Results: The fatty acids (FA), expressed in absolute values,not show any significant differences among the aforementioned groups. However, three were some trends which were confirmed when the FA were expressed in percentages. Thus, higher levels of docosahexaenoic acid (DHA) were found in SIG both in plasma and in the erythrocyte membrane, when expressed in percenta­ges. Eicosapentaenoic acid (EPA) was also higher in the SIG in the erythrocyte membrane, whereas in plasma the differences were of marginal significance. On the other hand、arachidonic and linoleic acids had lower values in the SIG in erythrocytes. The theoretical optimal intake of n-3 LC PUFA corresponded to a plasma concentration of 45.9 mcg/ml n-3 LC PUFA or 2.54% of the total fatty content (2.29% of DHA). The corresponding cut-offs in erythrocyte membranes were 7.54% of total lipids (5.59% of DHA). Conclusion: The best markers of n-3 LC PUFA intake were DHA for plasma and DHA and EPA for phospholipids, all of them expressed in proportions of total FA. The arachidonic and linoleic acids (in percentages) in erythrocyte phospholipids were also good markers of n-3 intake. This probably reflects the metabolic competition between both PUFA families.