Clearance methods, allowing for repeated use in the same animals, were applied in a cross-sectional study to establish the normal increase in renal function in rats. It was shown that the glomerular filtration rate (GFR) and the effective renal plasma flow (ERPF) per gram of kidney was low in the 4-week-old rats. Levels comparable to those of adult rats were reached at an age of about 7 weeks. From that age on the GFR and the ERPF, as well as kidney weight, increased at a lower rate than did the body weight. The present findings, obtained by these rather simple techniques are, in general, concurrent with data from the literature on the development and maturation of renal function in rats.