The natriuretic activity shown in earlier cross-circulation experiments to develop in the blood of rats undergoing sustained vascular expansion has been further characterized. Following whole blood infusion and urine reinfusion of the donor rat, a cross-circulated isovolaemic partner exhibits a natriuresis of gradual onset, requiring 60–80 min to reach a peak (0.04–3.24 μequiv./min per gram kidney weight). The gradual feature of recipient natriuresis was unchanged by infusing the donor rat 1 h prior to cross-circulation. Once developed, the recipient natriuresis was sustained when fluid depletion by the renal response was prevented, but was reversible with a half-life of about 30 min on interruption of the cross-circulation; reconnection of cross-circulation promptly restored recipient natriuresis. No significant natriuresis (0.04–0.16 μequiv./min per gram kidney weight) occurred in the recipients if the donors in comparable cross-circulations were not infused. These findings confirm that a natriuretic activity develops in the blood as a consequence of vascular expansion, and reveal that the activity has a slowly developing and reversible action on the renal natriuretic mechanism.