The effect of maternal caffeine infusion on uterine blood flow and fetal oxygenation was studied in chronically catheterized pregnant sheep equipped with electromagnetic flow probes. The systemic administration of caffeine in doses of 8 and 24 mg/kg of body weight resulted in peak maternal serum levels of 16.7 +/- 0.9 and 38.5 +/- 2.1 micrograms/ml and fetal serum levels of 10.9 +/- 0.7 and 34.9 +/- 4.5 micrograms/ml, respectively. While no significant effects were noted at the 8 mg/kg dose, administration of 24 mg/kg was associated with a 5% reduction in uterine blood flow (p less than 0.05) and a 7% increase in mean arterial pressure (p less than 0.05). No significant alteration in maternal or fetal oxygenation occurred at either dose. These findings suggest that any adverse effects on fetal development attributed to caffeine are not likely exerted by influencing uterine blood flow.