Starting from routine renal clearance measurements in a partially shielded whole-body counter, generally applicable algorithms were developed empirically which permit the reconstruction of the total course of retention up to 28 min p.i. from e.g. the 12 min and 24 min values only. By formal differentiation the instantaneous slope of the decline in retention and thus the renal clearance can be calculated employing the whole-body counting principle. Integration yields the possibility of subtracting background from renograms to calculate single renal clearances. The method offers advantages in cases of reduced measurement geometry and/or low count rates.