The renal resistive index (RI) value of 0.73 has been proposed as the upper limit in normal adult dogs. In humans, changes in RI with age are associated with plasma renin activity. There are relatively few equivalent reference data for dogs. We obtained reference RI data from 22 clinically healthy dogs <4 months of age and 33 healthy dogs between 4 months and 7 years of age. An association between the RI and plasma renin activity was investigated. The mean RI in the older dogs was 0.65 +/- 0.05 vs. 0.75 +/- 0.05 in dogs <4 months of age. The mean plasma renin activity in the older dogs was 1.18 +/- 1.03 vs. 4.23 +/- 3.09 ng/ml/h in dogs <4 months of age. There was a weak linear relationship between the RI and plasma renin activity (r2 = 0.280, P < 0.01) in dogs <4 months of age. Also in these younger dogs, RI was negatively correlated with age (r2 = 0.682, P < 0.01). The RI was higher in dogs <4 months of age than in older dogs. Therefore, the mean renal RI is slightly higher in young dogs than reported for an older population and interpretation of the RI must include an assessment of patient age.