Reliable monitoring of arterial blood gas values in the fetal scalp microcirculation requires an unimpaired local skin blood flow. Previous studies have shown that in the fetus blood flow to "nonvital" organs and tissues is reduced during hypoxemia. However, data on fetal scalp blood flow during hypoxemia have not been reported. Therefore, the influence of hypoxemia on scalp blood flow was studied in four chronically catheterized fetal lambs of 125-135 days of gestation. Red blood cell flow in the fetal scalp was continuously measured by the laser Doppler method during nine experiments. Reducing the oxygen saturation of fetal carotid arterial blood from an average control value of 52 to 21% for 20 min resulted in an average decrease of the laser Doppler flow index of 7%. In vitro studies revealed that more than half of the observed decrease in laser Doppler flow index could be attributed to enhanced absorption of light from the HeNe-laser caused by the deoxygenation of oxyhemoglobin itself. In the first 20 min after hypoxia, fetal scalp blood flow remained below the prehypoxic value. It is concluded that moderate hypoxemia causes only a small decrease in fetal scalp blood flow, certainly when compared to the dramatic fall in scalp blood flow that has been observed when local or circular pressure is exerted onto the fetal scalp.