Excretory function of one kidney of the rat was reduced acutely by either partial renal arterial occlusion, elevation of ureteral pressure or complete ligation of the renal pedicle. U_Na·V by the opposite kidney increased approximately twofold during partial arterial occlusion or elevated ureteral pressure and this was not associated with increased kidney or superficial nephron glomerular filtration rate (GFR). Absolute proximal reabsorption fell during contralateral arterial constriction, parallel to a drop in GFR. The calculated delivery of filtrate out of proximal tubules rose when natriuresis occurred during the experimental maneuvers and decreased parallel to sodium excretion when obstruction or clamping of the opposite kidney was released. Unilateral ligation of the renal pedicle did not result in changes in nephron filtration rate or proximal tubular reabsorption, although urine flow rate rose, attended by a smaller and nonsignificant natriuresis: in this latter circumstance a distal fall in Na transport may be postulated, while the former experimental maneuvers could have caused an increase in Na excretion by a proximal effect. Thus, contralateral natriuresis during partial unilateral reduction in renal function may be due to different factors, specifically related to the experimental maneuver used to lower GFR.