OBJECTIVE The direct effects of furosemide infusion have not been systematically examined in the fetus. Our objective was to explore the hemodynamic and urinary responses to a 4-hour infusion of furosemide into fetal sheep. METHODS Furosemide (0, 1, 5, or 10 mg/hr) was infused into the fetal inferior vena cava for 4 hours in 15 chronically catheterized near-term sheep. RESULTS Relative to vehicle infusion, furosemide produced dose-dependent increases in fetal arterial pressure (analysis of variance, p < 0.05 when comparing groups), fetal heart rate (p < 0.0001), urine flow (p < 0.0001), and urine osmolality, sodium, and chloride, concentrations (p < 0.0001). Concomitantly, there were dose-dependent decreases in fetoplacental blood volume, fetal plasma chloride (p < 0.0001) and fetal venous pressure (p < 0.05). The changes in urine flow rate and fetal arterial pressure were positively correlated (r = 0.46, p < 0.01), suggesting that the diuresis was mediated in part by fetal arterial pressure. The four-quadrant amniotic fluid index increased during the furosemide infusions but not during vehicle infusions (p = 0.022). CONCLUSIONS Furosemide infusion caused a marked dose-dependent diuresis in the ovine fetus that appears to be partly mediated by increases in vascular pressure. Although amniotic fluid volume increases and fetal blood volume decreases, the reduction in blood volume was small compared with the urine volume excreted.