The present study was carried out to investigate the effect of sodium intake on urinary renin excretion in rats. Renin activity was measured before and after acidification, by radioimmunoassay of angiotensin I generated by incubation with partially purified homologous renin substrate. The molecular weight of renin was determined by gel filtration. Low sodium intake for 4 weeks resulted in a 16-fold increase in urinary renin excretion and led to a 15-fold increase in plasma renin activity. A 2.5-fold increase in renal renin content was simultaneously observed. In contrast, high sodium intake for 4 weeks decreased urinary renin excretion, plasma renin activity and renal renin content to about 20%, 25% and 35% of the control level, respectively. No significant change in renin activity after acidification was observed in all the urine samples. The molecular weight of renin in the urine from rats on low or high sodium intake was 40,000, the value being identical with that from control rats. These results indicate that urinary renin excretion is well correlated with plasma renin activity, and that only active renin with a molecular weight of 40,000 is excreted in the urine of rats, even under conditions of altered sodium intake.