1. The anatomy of the renal lymphatics and the flow and composition of renal lymph have been investigated in sheep. Lymphatic capillaries were demonstrated in the cortex and cortico-medullary areas of the sheep's kidney but not within the medulla. The lymph formed in the sheep's kidney drains from the hilum through several small lymphatics; there is no lymphatic drainage from the renal capsule.2. Lymph flow from the sheep's kidney varied from 0.5 to 3.0 ml./hr. The concentrations of electrolytes and urea in renal lymph were found to be similar to lymph from other regions of the body, but the concentration of endogenous creatinine was lower. The average concentration of protein in renal lymph was 43% of the plasma levels; there was a significantly higher proportion of albumin in renal lymph than in plasma. When labelled albumin was injected intravenously, the specific activities of the plasma and renal lymph albumin equilibrated in about 2 hr.3. When [(14)C]inulin, [(125)I]hippuran or [(14)C]creatinine were infused intravenously radioactivity appeared rapidly in the renal lymph. When steady-state levels were reached in the circulating plasma, the renal lymph/jugular vein plasma ratios for [(14)C]inulin and [(14)C]creatinine were 0.82 while for [(125)I]hippuran the ratio was 0.34. It was considered that the concentration of these substances in renal lymph was similar to the concentration in renal vein plasma.4. The concentration of renin enzyme was on the average about 8 times higher in renal lymph than in jugular vein or renal vein plasma or in lymph from the hind limbs.5. Renal lymph appeared to be formed principally, if not entirely, in the renal cortex and appeared to be a modified filtrate derived from post-glomerular blood capillaries. If any contribution to renal lymph does come from the medulla, it must be small in relation to the volume of lymph formed in the cortex.