Six singleton fetal sheep of 118 - 122 days gestational age were instrumented with flow sensors on the brachiocephalic artery, the postductal aorta, and the common umbilical artery and with arterial and venous intravascular catheters. At 125 - 131 days of gestation, we started week long continuous recordings of flows and pressures. After control measures had been obtained, the fetuses were given continuous i.v. infusions of adult sheep plasma at an initial rate of 229 ml/day. After one week of infusion, fetal plasma protein concentrations had increased from 34 to 78 g/liter, arterial and venous pressures had increased from 42 to 64 and from 2.7 to 3.7 mm Hg, and systemic resistance (exclusive of the coronary bed) had increased from 0.047 to 0.075 mm Hg min/ml, while placental resistance had increased from 0.065 to 0.111 mm Hg min/ml. Fetal plasma renin activities fell as early as one day after the start of infusion and remained below control. (All changes P<0.05). All flows decreased slightly although these decreases were not statistically significant. Thus, the increase in arterial pressure was entirely due to an increase in systemic and placental resistances.