Various renal functions were studied in 45 young Koreans, 41 males and 4 females. The glomerular filtration rate, the renal plasma and blood flows and the filtration fraction were comparable to those in the occidental. The renal diluting capacity, as judged by the value of positive free water clearance (C_H2O), was also comparable to that in the occidental. However, the renal concentrating ability, as judged by values of both the maximum urine osmolality induced by pitressin injection or during dehydration and the maximum negative free water clearance (T^c_mH2O) during mannitol diuresis, was somewhat low in the Korean. The tubular reabsorptive capacity of glucose (TmG) Was significantly low in the Korean while the tubular secretory capacity of PAH (TmPAH) was not altered. The urea clearance and its ratio with the inulin clearance were generally low. Moreover, the normal osmolar composition of urine indicated that urea is responsible for only 20 to 30%, while salt accounts for nearly 2/3 of the total urine osmolality. In view of the fact that the Korean is chronically maintained on a low or poor protein diet, these results are discussed in the light of this peculiar dietary habit of the Korean. Although the reduction of both the renal concentrating ability and the urea clearance could be reasonably attributed to a low protein intake, the observed low value of Tmc could not be explained thus at present.