Sustained fetal hyperglycemia was produced in eight chronically catheterized fetal lambs (seven twins, and singleton) by means of direct fetal glucose infusion. In twin preparations, only one twin was infused, the noninfused twin serving as a simultaneous in utero control. Glucose infusions lasted 7.6 +/- 1.8 days and resulted in significant fetal hyperglycemia (from 20.3 +/- 1.1 mg/dl to 58.2 +/- 4.7 mg/dl, P less than 0.001). The magnitude of the hyperglycemia was linearly related to the glucose infusion rate. Elevations of fetal plasma glucose and glucose infusion rate were associated with a significant fall in fetal arterial oxygen content (P less than 0.001). In twin preparations studied, these relationships remained when the simultaneously sampled, noninfused twin was used as control. The fetal glucose-induced hypoxemia was not associated with fetal acidosis (tissue hypoxia) until the arterial oxygen content fell below 30% of baseline (mean base deficit in acidotic fetuses = 11.2 +/- 2.2 meq/liter). Although Pa02 fell in hypoxemic fetuses (from 13.5 +/- 1.2 mmHg to 9.7 +/- 1.2 mmHg), the difference was not significant. Fetal plasma insulin rose during hyperglycemia from 10.2 +/- 3.1 micro U/ml to a peak concentration of 26.2 +/- 3.3 micro U/ml, but this response was blunted in markedly hypoxemic fetuses. Neither fetal anemia nor hemoconcentration were evident in these preparations to account for the fall in fetal oxygen content.