On the question of body fluid volume or sodium status influencing renin release.

1. The extent to which renin release is affected by simultaneous changes in body Na and body fluid volume was studied in six sheep. 2. The animals' water intake was restricted for 10-17 days after which they were offered solutions containing varying amounts of NaCl. 3. Plasma renin concentration (PRC) of water restricted sheep was 2-3 times normal. 4. The changes in PRC following drinking were inversely related to the amount of sodium consumed, Na excretion and plasma Na concentration. There was no correlation between the changes of PRC and of plasma volume in so far as the latter is reflected by alterations in plasma protein concentration. 5. We conclude that changes in renin release were related to the animals' handling of NA, and not to alterations in body fluid volume. 6. These findings are compatible with the proposition that renin release was mediated by a macula densa mechanism.