The effect of maternal acidosis on fetal acid-base balance was studied in a dual circuit perfusion of a single cotyledon in normal, term, human placentas. Both the fetal and maternal (intervillous) circulations were perfused with a Krebs-Ringer solution adjusted to pH values between 7.35 and 7.45. After a control period, the perfusate in the maternal circulation was replaced by an acidified medium (mean pH 7.06) for 30 min. This was followed by a second control period of 30 min during which the acidified maternal perfusate was replaced with the original medium. During the 30 min of maternal acidosis, fetal vein pH was not significantly altered despite the large decrease in maternal artery pH, but there was an efflux of total CO2 (tCO2) from the placenta into the maternal circulation which was not matched by an influx of tCO2 from the fetal circulation. The tCO2 transferred was in the form of bicarbonate rather than dissolved CO2, but the maximal rate of tCO2 transfer of in the form of bicarbonate was lower than the rate of placental transfer of tCO2 necessary in vivo. It is probable therefore that bicarbonate does not play a major role in placental CO2 transfer but the placental tissue bicarbonate pool may play an important part in buffering the fetus against changes in maternal pH or blood gas status.