A sensitive radioimmunoassay is described for the measurement of plasma concentrations of canine growth hormone (cGH) as low as 0.25 ng/ml. The assay utilizes enzymatically iodinated cGH and the double antibody technique. The mean plasma concentration of growth hormone in the normal dog after overnight fast is 1.75 plus or minus .17 ng/ml. Exogenous cGH was cleared from the plasma of both the normal and hypophysectomized dog with a mean half-life of 25.6 plus or minus 1.0 min and was distributed in a volume equal to 8.9% of the body weight. Insulin hypoglycemia produced a 3- to 5-fold increase in plasma GH in 4 of 6 dogs and arginine infusion failed to produce a statistically significant rise.