51Cr-EDTA clearance was determined in 50 children from the activity in one plasma sample by means of two methods. The calculation of 51Cr-EDTA clearance by method I (ClI) was based on a transformation of the biexponential time-activity curve into a monoexponential with identical area below the time-activity curve. The calculation of 51Cr-EDTA clearance by method II (ClII) was based on a close relation between 51Cr-EDTA clearance and the reciprocal apparent volume of distribution normalized with respect to body surface area. It was demonstrated that the two methods were closely related. To test the accuracy of the two methods, the results of ClI and ClII were compared to 51Cr-EDTA clearance calculated by means of a standard, multiple plasma sample method (ClSM). The correlations between ClSM and ClI and between ClSM and ClII were almost identical (r = 0.993 and r = 0.992). The accuracy of 51Cr-EDTA clearance calculated as ClI and ClII did not differ significantly from each other.