1. Blood pressure, plasma renin activity, plasma aldosterone, urinary noradrenaline during sleep (UNA-S) and several estimates of sodium intake were determined in 379 normotensive subjects (age 13–70) to investigate the relationship of these variables to blood pressure.

2. Blood pressure was correlated with age, weight, plasma renin activity UNA-S, and estimates of sodium intake. These variables were frequently intercorrelated.

3. Multiple-correlation analysis revealed that after removal of the effects of age, blood pressure was related to weight, plasma renin activity, UNA-S and estimates of sodium intake.

4. However, multiple-regression analysis failed to demonstrate an effect of plasma renin activity, UNA-S, or estimates of sodium intake on blood pressure when the effects of age, weight, race and sex were removed.

5. Careful matching of subjects by age, weight, race and sex in studies of blood pressure and biochemical factors in normal subjects is crucial to proper interpretation of such data.

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