1. 24 h aldosterone secretion rates (ASR) have been measured in six normal volunteers while recumbent all day and while standing for 12 h, on 200 and 10 mmol/day sodium diets and after salt-poor albumin infusions (75 g in 150 ml), which significantly expanded plasma volume.
2. The mean ASR on the 10 mmol/day sodium diet, both without and with the salt-poor albumin infusion, was highly significantly increased above the mean ASR on the 200 mmol/day sodium diet, both in the recumbent and in the upright posture.
3. There was no significant difference between the mean ASR values on the 10 mmol/day sodium diet alone and after the infusion of albumin either in the recumbent or in the upright posture.
4. The above observations suggest that sodium deprivation raises ASR by a mechanism or mechanisms unrelated to plasma volume.