Umbilical blood flow rate (UBF) was measured in 9 exteriorized fetal lambs by weight changes of the fetus during total umbilical vein occlusion. Concentrations of oxygen, lactate and pyruvate in umbilical arterial (UA) and umbilical venous (UV) blood were determined. Rate of fetal oxygen consumption (.QO2) was 5.2 +/- (SEM) 0.48 ml. kg(-1) . min (-1), and was correlated with UBF (r = 0.793), but not with the content of O2 in UA or UV. The UV-UA differences in lactate/pyruvate ratios indicated varying degrees of anaerobic metabolism in those fetuses with highest UBF and .QO2, but not in poorly perfused fetuses with low .QO2. Fetal exteriorization can result in inadequate perfusion of fetal tissue, lowering of fetal oxygen consumption and some anaerobic metabolism.