1. The mineralocorticoid activity of 18-hydroxycorticosterone (18-OH-B) has been compared with that of aldosterone by using human bioassay in vivo with measurement of rectal potential difference and urinary log 10 Na+/K+ ratio.
2. A log-linear relationship was found between maximum change in rectal potential difference and increasing doses of aldosterone.
3. No mineralocorticoid activity could be demonstrated after an intravenous bolus and infusion of 18-OH-B.
4. The half-life of clearance of 18-OH-B was measured in three subjects and found to be 28, 48 and 24 min.