Cystatin C is a low-molecular-mass acid protein produced at a constant rate by all nucleated cells and cleared by glomerular ¢ltration. In human medicine it is considered to be a better indicator of renal failure than creatinine. Plasma (Pl-) cystatin C measurements in 179 clinically healthy dogs, using an immunoturbidimetric procedure for human cystatin C, showed a Gaussian distribution with an upper limit of 1.3 mg/l. There were no di¡erences between the sexes. Pl-cystatin C was slightly lower in 1^8-year-old adults than in younger or older dogs. It was also lower in dogs weighing less than 15 kg than in heavier ones. Meals produced a dramatic decrease in Pl-cystatin C that lasted for up to 9h. Pl-cystatin C was elevated in 98% of dogs with renal insu⁄ciency, even in some cases where the Pl-creatinine was normal. Cystatin C may therefore be a useful indicator of renal insu⁄ciency in clinically relevant dogs with borderline P1-creatinine values.