The placental and milk transfer of chloroquine was studied in seven subjects and six subjects, respectively. Seven pregnant women at term were administered chloroquine phosphate (5 mg/kg) i.m. The maternal blood and arterial and venous umbilical cord blood were obtained during delivery. The maternal chloroquine blood levels varied from 0.438 to 1.193 micrograms/ml. The venous cord blood ranged from 0.607 to 0.869, whereas the arterial levels ranged from 0.480 to 0.905 micrograms/ml. Six subjects who were administered chloroquine phosphate (5 mg/kg) postpartum had chloroquine milk levels of 0.192-0.319 micrograms/ml. The milk:blood ratio ranged from 0.268 to 0.462. Chloroquine readily crossed the placenta and was excreted into the breast milk.