1. Theories have been developed to try to interpret the effects of finite time of equilibration between plasma and body water, and the mechanism of renal function, on the variations of activity and specific activity of urea in plasma and urine after initial injection. 2. The magnitudes of errors likely to arise through approximations made in estimating the pool of body urea etc. have been derived. 3. Experimental results do not fit exactly with extreme models postulated, but are usually intermediate.