The effects of renal lymphatic occlusion or increased lymph flow due to renal vein constriction on renal function were investigated in rats. In each experiment, the renal lymphatics or vein of the left kidney were occluded or constricted and the right kidney served as a control. Occlusion of renal lymphatics caused renal enlargement, no change in glomerular filtration rate, a marked increase in urine flow and solute excretion without any change in urine osmolality, and enhanced urinary loss of urea, potassium, sodium and ammonium. Urea concentrations in medullary and papillary tissues were significantly elevated. Renal vein constriction caused renal enlargement and a marked drop in glomerular filtration rate, urine volume, urine osmolality and solute excretion. tissue concentrations of urea and potassium were decreased in the medulla and papilla and total tissue solute was significantly decreased in the papilla. The data indicate that in the rat, renal lymphatic occlusion traps urea in the medulla and induces a urea diuresis resulting in a large flow of normally concentrated urine. On the other hand, increased lymph flow secondary to renal vein constriction decreases medullary urea and potassium concentrations and papillary osmolality. These changes and the reduced glomerular filtration rate result in a small flow if dilute urine. Thus both renal lymphatic occlusion and enhanced lymph flow have a significant effect on renal function.