Samples were obtained from Dutch mothers and their babies throughout the perinatal period and analyzed by gas-liquid chromatography with electron-capture detection for a range of organochlorine pesticides. Organochlorine concentrations in the blood of breast or bottle-fed babies were not significantly different. Differences in organochlorine concentrations in blood between mothers who had "slimmed" and those who had not were small, but it must be noted that there were few slimming mothers in our sample. Twelve to 21% and 36-61% of the estimated daily intake of dieldrin and DDT, respectively, by mothers may be eliminated by lactation. The placenta restricts the transmission of organochlorine pesticides to the fetus to some extent. The very low concentrations of organochlorine in fetal blood prevented a study of the comparative metabolism in mothers and babies.