Placental transfer of 14C-dimethyltubocurarine (14C-dmtc) was studied during Caesarean section. Liquid scintillation counting was used for determination of 14C activity from the maternal and foetal blood samples. Paper chromatography was used to confirm the stability of 14C-dmtc. In all cases 14C-dmtc was able to cross the placenta. Detectable amounts of 14C-dmtc were found in umbilical blood 2 min after injection into the mother. Six and 10 min after injection, the 14C concentration in the umbilical blood was 12% of the corresponding maternal value.