This study was carried out to investigate the effect of fetal hypoxemia, as a result of acute intermittent maternal hypoxia, on the switchover from fetal to adult type Hb as well as Hb oxygen affinity and 2,3-diphosphoglycerate levels in the near-term fetus. These experiments were carried out using 10 fetal lambs with gestational ages ranging from 132 to 140 d. After the installation of appropriate fetal catheters, five of the ewes were exposed to an air mixture containing 10% O2 for 90 min/d for 4 consecutive d. Blood samples were withdrawn before the beginning of the hypoxic experiments and between the 5th and 6th d after the first episode of hypoxia. These samples were for the determination of 2,3-diphosphoglycerate concentration, arterial O2 pressure at which Hb is 50% saturated, and Hb type synthesis. Blood gases were monitored during each hypoxic episode. During the hypoxia, fetal arterial O2 pressure decreased from 2.43 +/- 0.36 kPa (18.2 +/- 2.7 mm Hg) to 1.57 +/- 0.17 kPa (11.8 +/- 1.3 mm Hg). These values returned to their initial levels after cessation of the maternal hypoxia. Five control animals of the same gestational age were also followed. During the interval of the study, a decrease of fetal Hb synthesis was noted (71.7 +/- 12.1 to 57.4 +/- 10.2%, p less than 0.001) in the control group. However, the level of fetal Hb synthesis did not significantly change in the hypoxic group (85.1 +/- 11.1 versus 80.6 +/- 18.9%).(ABSTRACT TRUNCATED AT 250 WORDS)