We studied the pattern of plasma active renin concentration (ARC), prorenin concentration (PRC), renal renin concentration, and the renin mRNA levels in ovine fetuses subjected for 24 h to reduced renal perfusion pressure (RPP). The results obtained in five animals (133.8 +/- 1.4 days of gestation) in which RPP was reduced by 10 mmHg were compared with those in seven control fetuses (130.3 +/- 0. 8 days of gestation) without pressure reduction. Plasma samples were obtained before and at intervals of 24 h after initiating reduced RPP. The plasma ARC increased within 60 min of reduced RPP, reaching a maximum (13.0 +/- 4.7 vs. 115.7 +/- 23.8, P < 0.01) at 3 h. The ARC then declined toward control values. In contrast, plasma PRC did not increase consistently until 4 h into reduced RPP, with maximal levels at 24 h (8.2 +/- 2.4 vs. 87.7 +/- 21.9, P = 0.016). Within the kidney PRC, but not ARC, increased significantly, by 2.5-fold. Reduced RPP also increased renal renin mRNA levels (P = 0.004). We conclude that a chronic reduction in RPP in the near-term ovine fetus increases renal PRC and is associated with increased plasma prorenin levels. The data suggest that the conversion of prorenin to active renin is an important regulation point of the renin ANG system during development.