Immunoreactive FSH and LH, testosterone and cortisol were measured in venous plasma samples obtained every 4 hr for 24 hr from 8 healthy young men. Subjects were studied on 2 occasions, one week apart, once with and once without dexamethasone treatment, the order randomly assigned. On no treatment, FSH, testosterone and cortisol each showed a diurnal cycle with concurrence of highest mean levels at 8:00 hr and lowest mean levels between 24:00 and 4:00 hr. The relative amplitudes3 of the cycles were: cortisol +102, −83%; testosterone +13, −20%; and FSH +6, −5%. No cycle in LH was demonstrated. Dexamethasone administration resulted in marked suppression of cortisol levels with loss of cyclicity, elevation of LH levels, and no change in over-all levels or cyclicity of FSH and testosterone. The results suggest that FSH and testosterone levels are linked together, but the mechanism is not known. The reason for the elevation of LH levels by dexamethasone is unclear.