Several lines of research suggest that renal lymph is formed totally, or in part, as a filtrate of postglomerular blood. Even so, renal interstitial fluid dynamics is necessarily dominated by the rapid transfer of large volumes of reabsorbate from tubular lumen to peritubular capillary. To determine the effects of tubular reabsorption on lymph formation, renal lymph flow and composition were studied before and during alterations in tubular reabsorption produced by diuresis. A 25% reduction in whole kidney fluid reabsorption rate did not alter renal lymph flow or lymph protein concentration. A concomitant decrease in plasma protein concentration, however, suggests that any deficit in lymph flow resulting from decreased reabsorbate content may have been obscured by an increase in vascular filtration. An increase in renal lymph PAH and creatinine concentrations relative to those of arterial and renal venous blood plasma supports this conclusion. It appears that renal lymph flow may be determined by both vascular filtration and tubular reabsorption.