The glomerular filtration rates (GFR) of 18 dogs of different breeds were estimated by simultaneously comparing the plasma decay curves of 99M-Tc-diethylenetriaminepentaacetic acid (Tc-DTPA) and the contrast medium iohexol. Nine of the dogs were clinically healthy, and nine had renal disease. After the intravenous injection of the two substances, nine to 12 blood samples were taken during the first six hours and in some cases up to 24 hours. The total clearance of the substances was determined by one- and two-compartment models. Individual GFRiohexol values, estimated by using a two-compartment model in 16 of the dogs, ranged from 39.0 ml min-1 m-2 to 84.3 ml min-1 m-2 and for GFRTc-DTPA from 41.1 ml min-1 m-2 to 106.6 ml min-1 m-2. The GFRiohexol values estimated by using a one-compartment model in the 18 dogs ranged from 4.52 ml min-1 m-2 to 105 ml min-1 m-2 and for GFRTc-DTPA from 5.4 ml min-1 m-2 to 118.9 ml min-1 m-2. The agreement between the two methods was good, as shown by a limits of agreement plot. The ratio of GFRTc-DTPA/GFRiohexol was 1.15 throughout the range of measurement (SD = 0.085). The study indicates that the iohexol method is a valid measure of GFR in the dog. The practical aspects make it a useful tool in small animal practice.