1. Water and electrolyte excretion has been measured in both the follicular and luteal phases of the menstrual cycle in eight normal females undergoing a sustained water diuresis.
2. When the subjects were supine, no differences were found in urinary excretion during the follicular and luteal phases.
3. When the subjects were tilted to the upright position, the urine volume, excretion of sodium and potassium and the osmolar and free water clearances fell, suggesting an alteration in proximal tubular function. These changes were statistically greater in the luteal than in the follicular phase of the cycle.
4. It is postulated that the enhanced retention of electrolytes and water in response to tilting in the luteal phase is a compensatory mechanism for the decreased effective blood volume at that time.