Accretion of polyunsaturated fatty acids was examined in human fetal brain and the corresponding placenta from 20 pregnancies covering the gestational ages 9 to 22 weeks to provide data for the first and second trimesters and to complement existing data for the third trimester and early postnatal periods and also to investigate any correlation between the levels of polyunsaturated fatty acids in the developing brain and placenta. Brain polyunsaturated fatty acid (PUFA) accretion was evident in the earliest samples. During this early phase of gestation, a gradual increase in brain PUFA levels was noted. Levels of PUFA in the placental samples were stable during this period. Comparison of PUFA levels from each brain revealed no correlation with PUFA levels in the corresponding placenta. PUFA accretion in human fetal brain in the second trimester, the period of rapid proliferation of neuronal and glial elements, coincides with the onset of ganglioside accumulation. Although this study provides no direct evidence, finding stable levels of PUFA in placenta is consistent with the current concept that, in the human, the placenta is the principal source of PUFA for export to the fetus throughout gestation.