The average renin substrate concentration in 15-day-old female and male Wistar rats was about 300, expressed as ng angiotensin per ml. With increasing age it rose rapidly to a maximum of about 400 ng in females, and more slowly to about 700 ng in males. Castration of mature or immature rats of both sexes did not influence either renin substrate or the degree of its increase following nephrectomy. The average postbinephrectomy values was about 3100 ± 1000 ng in both sexes, being about 9 times increased in mature females and about 6 times in mature males. In immature females the postbinephrectomy values had increased by about 8 times, up to about 2400 ng. Renin substrate seems to be markedly influenced by changes in the environment.