The placental transfer of cimetidine 200 mg intravenously was investigated in 16 patients in normal labour and 40 patients undergoing elective Caesarean section. Cimetidine crosses the placental barrier, blood levels at delivery in mothers and infants being lower with increasing time from injection. Umbilical cord blood cimetidine levels are markedly lower than those of the mother during the first hour following administration but thereafter levels are similar. The mean fetal-maternal ratio at delivery was highest (0.84) at 1 1/2-2 hours following administration. Postdelivery infant and maternal blood samples showed that cimetidine could not be detected, in most cases, 19 hours following administration. The relevance of these findings is discussed.