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%.