Chloroquine, its N-dealkylated metabolites, and chloroquine N-oxides were detected in the urine of pregnant women who were receiving chloroquine medication whereas chloroquine and its nonpolar metabolites, desethyl- and didesethylchloroquine and 7-chloro-4-aminoquinoline, have been found in the neonates' urine, blood, and cord blood. We used thin-layer chromatography to separate chloroquine and its metabolites after their extraction from biological fluids. These compounds were identified by comparing their chromatographic and ultraviolet spectrophotometric characteristics with those of reference compounds. That chloroquine and its relatively nonpolar metabolites (including one without the alkyl side-chain, 7-chloro-4-aminoquinoline) cross the placenta is demonstrated by the presence of these compounds in the cord blood, neonatal systemic blood, and neonatal urine. The selective transfer of the compounds in the cord blood, neonatal system blood, and neonatal urine. The selective transfer of the compounds across the placenta and the clinical implications of these findings are discussed.