The physiologic events which may occur with the gravid women in the supine position suggest that uteroplacental blood flow may be impaired as a consequence of decreased venous return to the heart and the resulting decrease in cardiac output. Since the metabolic clearance rate of dehydroisoandrosterone sulfate (MCRDS) previously has been shown to reflect alterations in uteroplacental perfusion, the MCRDS was measured in a variety of different obstetric conditions during lateral and supine recumbency. No significant difference in the MCRDS was induced by this specific positional change, suggesting that uteroplacental perfusion was not altered. Experimental data are discussed which support this observation, and a possible mechanism for maintenance of uteroplacental blood flow under these circumstances is presented.