1. The 24 h urinary excretion of free cortisol and aldosterone-18-glucuronide has been studied in twenty-three control and twenty-three grossly obese subjects by a double-isotope-dilution derivative method.

2. Day-night variation of plasma and urinary cortisol has also been investigated.

3. The mean absolute steroid values were similar for the obese and non-obese subjects.

4. The 24 h urinary values for free cortisol expressed as μg/kg of body weight were significantly higher for the controls.

5. The majority of the obese and control subjects had a day-night variation of plasma cortisol. All subjects, with two exceptions, showed day-night variation of free cortisol in the urine.

6. It is suggested that the estimation of unconjugated cortisol in timed urinary collections could be useful for investigations of adrenocortical function.

7. The findings do not support the presence of increased adrenocortical activity in developed obesity.

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