1. Basal plasma renin activity, active and inactive plasma renin concentration were measured in 89 healthy recumbent children aged between 1 week and 16 years. 2. A significant ( P <0.001) age-related decrease for active ( r = −0.60), inactive ( r = −0.59) and total renin concentration ( r = −0.66) was observed. 3. After correction for the influence of age, active renin concentration correlated with plasma renin activity ( r = 0.81), but not with inactive renin concentration ( r = 0.18). 4. The proportions of active and inactive renin were not related to age, and the overall percentage of inactive renin was 79%.