Glomerula filtration rate (GFR), plasma creatinine concentration (CR), and plasma urea nitrogen concentration (BUN) were measured in 129 adult dogs with reduced renal mass. A preliminary examination of the relationship between CR and GFR was conducted, and the inverse model (GFR vs. 1/CR) was chosen for further evaluation. The slope of the regression of GFR on 1/CR which was computed from actual data was not statistically different from a theoretical regression line generated from the clearance equation. Evaluation of subsets of the population revealed no significant difference between male (n = 69) and female (n = 60) dogs on the slope of the regression equations. Diets differing in protein concentration (16% protein, n = 35: 21% protein, n = 75: 32% protein, n = 19) did not cause a significant difference in the slope of the regression equations. The regression equation and the confidence intervals generated in this study may be used to predict a probable range of GFR values from CR in individual dogs. Such values may be useful in adjusting drug dosages in dogs with renal disease. However, since the derived equation did not differ significantly from the theoretical inverse relationship between GFR and CR, it remains to be established whether the equation is advantageous.