Individual kidney glomerular filtration rate (IKGFR) was calculated from the upslope of the 99mTc-diethylenetriamine pentaacetic acid (DTPA) renogram in ten children with a normal solitary kidney (on the right in five and on the left in five). By using a method of renogram analysis that is independent of the intravascular (IV) component of background (BG) activity, the IV and extravascular (EV) components of background were separately quantified in various regions in proximity to the nephrectomy site. The size of the IV compartment was expressed as an absolute plasma volume (PV-BG), and that of the EV compartment was expressed as a "GFR equivalent," i.e., the undirectional rate of fluid transfer from the IV to EV spaces in ml per min (GFR-BG). The region areas from which these parameters were calculated were normalised to the area projected by the solitary kidney. In a region centred over the lower pole of the imagined absent kidney, PV-BG was 129 +/- 7 ml (SE) and GFR-BG was 28 +/- 1.8 ml per min. The corresponding values centred over the liver/spleen on the nephrectomised side were 353 +/- 24 ml and -2 +/- 2.9 ml per min, respectively. PV-BG, but not GFR-BG, was significantly higher over the liver (397 ml and -6.8 +/- 1.5 ml per min) compared with the spleen (287 ml, P less than .05; and 4.5 +/- 3.2 ml per min, P greater than .05). PV-BG and GFR-BG in a region placed over the centre of the nephrectomy site were intermediate between those superior and inferior: 231 +/- 17 ml and 15 +/- 2.7 ml per min, respectively. The IV signal rises while the EV falls during the second phase of the renogram. In the analysis of the renogram during this phase, therefore, background should be based on both supra- and subrenal regions of interest.