Post-natal development of single nephron glomerular filtration rate, superficial proximal tubular length, nephron number and kidney weight have been studied in Sprague Dawley and in Wistar rats. 2. Superficial tubular length is a non-linear function of body weight or age. There seems to be a rapid growth until animals weigh about 150 g in Wistar rats. In this strain, growth is slower thereafter. This difference is not as evident in Sprague Dawley rats. 3. Nephron numbers increase over the same period at which rapid tubular growth occurs. 4. Sprague Dawley rats have somewhat fewer, but longer, proximal tubules than do Wistar rats. 5. In all animals weighing more than 100 g, SNGFR is linearly related to weight. For younger, smaller Sprague Dawley rats, the same linearity holds over the age range studied--older than 20 days of age. In Wistar rats, SNGFR relative to weight is less in young animals. 6. By relating SNGFR to total kidney GFR, evidence is obtained that maturation of renal function also involves a greater increase in filtration by superficial than by juxtamedullary nephrons.
