The plasma clearances of technetium-99^m-labelled DTP A ([⁹⁹Tc^m]-DTPA) and the non-ionic contrast medium iohexol were estimated in 11 children with chronic renal failure for determination of the glomerular filtration rate (GFR). Equal values were obtained with the two substances provided plasma sampling was simultaneous, but when plasma was sampled within 3.5 h after injection of iohexol and [⁹⁹Tc^m]-DTPA the GFR was overestimated by more than 50%. For clearance values below 20 ml min^-1 1.73 m^-2, valid GFR estimates were obtained both from two plasma samples taken 3 h and 24 h after the injection of iohexol and from a single plasma sample taken 24 h after the injection.