1. Water and electrolyte excretion was measured in the follicular and luteal phases of the menstrual cycle in seven patients with idiopathic oedema and in four post-menopausal patients with this condition.
2. In contrast to previous findings in healthy women, the reduction in urinary flow and sodium excretion on being tilted to the upright position was not significantly different in the follicular and luteal phases in pre-menopausal patients and there were no significant differences between pre- and post-menopausal patients.
3. The percentage increase in packed cell volume on standing was significantly greater in patients with idiopathic oedema than in normal subjects in the luteal phase of the menstrual cycle.
4. It is postulated that the enhanced retention of electrolytes and water in response to tilting in patients with idiopathic oedema is a compensatory mechanism for the decreased effective blood volume at that time.