In a retrospective investigation we found that the endogenous creatinine clearance (Ccr) of 35 nephrectomized patients was lower than that of normal people of comparable age with two intact kidneys. The Ccr level in the post-nephrectomy patients appeared to correlate with the age of the patients at the time of nephrectomy. The average Ccr value was 99.2 ml/min in the younger group, and 54.9 ml/min in the older group. Furthermore, we also found that the measurement of the remaining kidney in the younger group was larger than that of the matched control group, but no difference was found between the measurement of the remaining kidney in the older group and that of the matched control group. Therefore, we suggest that the remaining kidney in patients over 50 years of age may be described as "a low compensatory kidney". The results from an experimental study of Wistar rats indicated that the dry and wet weights of the remaining kidneys of the younger rats which had undergone neptrectomy were 1.53 X 10(-3) g/gwt, and 5.89 X 10(-3) g/gwt respectively, vs 1.05 X 10 g/gwt and 3.82 X 10(-3) g/gwt in older ones. In tissue culture using serum of younger nephrectomized rats, the amount of 3H-TdR incorporation into the monolayer cells of renal cortex was 1,405.4 CPM, whereas it was 1,025.4 CPM by using serum of older nephrectomized rats. In addition, the cells of renal cortices from both younger and older rats had a similar response to the same serum from nephrectomized rats.(ABSTRACT TRUNCATED AT 250 WORDS)