1. In order to investigate the role of circulating serum factors in the altered renal haemo-dynamics and enhanced renal tubular transport observed in renal growth, micropuncture experiments were performed on normal animals infused with 20% plasma derived from animals in whom unilateral nephrectomy had been performed 3 days previously. 2. When animals infused with plasma from uninephrectomized animals (NxP) were compared with those infused with control plasma, the former had a higher tubular fluid flow rate measured at both the late proximal (LP; 26-7 + 1P6 vs. 18i4 + 1P4nl min-'; P< 0-001) and early distal (ED; 14 9 + P 0 vs. 4. Tubular Nae transport was higher in the animals infused with NxP as evidenced by a decrease in the fractional delivery of Nae at the ED site (4 5 + 0A4 vs. 6-5 + 0-6% of the filtered load; P< 0 05). However, in the final urine there was a significant increase in the urinary sodium excretion in animals infused with NxP (0-67 + 0-14 vs. 0-29 + 0 09%; P < 0 05) indicating that natriuresis and probably diuresis was a result of inhibition of Nae and water transport in the late distal tubule and collecting duct. 5. In conclusion, these experiments demonstrate that circulating factors induced by a reduction in renal mass significantly alter glomerular filtration and tubular Nae transport.