The hypothesis that endogenously released adenosine plays an important role in maintaining patency of the fetal lamb ductus arteriosus was tested. The design of the study was (1) to determine the effect, if any, of exogenous adenosine on blood flow through the ductus arteriosus and (2) to evaluate the relationship among the partial pressure of oxygen in arterial blood, circulating endogenous plasma adenosine concentration, and the rate of blood flow through the ductus. When exogenous adenosine (5 mumoles) was administered during oxygen-induced ductal constriction, ductal blood flow increased from 101 +/- 6 ml/min to 153 +/- 4 ml/min (p less than 0.01). When fetal blood adenosine concentrations were measured during nonventilation and ventilation with 100% oxygen, endogenous adenosine concentrations fell to less than one-half of the preventilation levels, i.e., from 1.12 +/- 0.17 to 0.49 +/- 0.03 microM (p less than 0.01). Finally, when fetal lambs were ventilated with increasing concentrations of oxygen (0%, 10%, 20%, 60%, and 100%) and measurements obtained simultaneously at each level, there was a significant monoexponential relationship among the rise in PO2, the fall in plasma adenosine concentration, and the decrease in ductal blood flow. These data suggest that: (1) adenosine is a potent vasodilator of the lamb ductus arteriosus during oxygen-induced vasoconstriction; (2) fetal endogenous plasma adenosine levels fall significantly when PO2 is increased; and (3) the fall in adenosine concentrations parallels a decrease in ductal blood flow. The findings suggest that the endogenous vasodilator adenosine plays an important role in maintaining ductal patency in utero.