Abstract Totally isolated canine kidneys were perfused at 37°C. for 240 minutes with whole blood from dogs that had undergone a sham operation or a unilateral nephrectomy 24 hours Previously. Renal blood fl ow, glomerular filtration rate, serum sodium, and serum osmolality mere not significantly different in the experimental and control preparations. Nucleic acid synthesis as measured by 3H-adenine incorporation was 75 percent higher (p < 0.01) in the kidneys perfused with blood from the neprectomized dogs. This result supports the theory that a blood-borne factor stimulates nucleic acid synthesis and initiates compensatory renal growth.