The renal excretion of renin infused intravenously was studied in anesthetized rats. The renin in the urine was adsorbed onto DEAE cellulose and selectively eluted for separate assay on the rat blood pressure preparation. The influence of saline loading, furosemide, mercuric chloride, and maleic acid was determined by following the changes in renin excretion. Both mercuric chloride and maleic acid led to a sixfold increase in the amount of renin appearing in the urine. Among the various factors which might have led to this result, an effect on renal tubular reabsorption after glomerular filtration of the renin is the most likely.