The effect of the serotonin antagonist, cyproheptadine (Cypro), on metyrapone-induced stimulation of the pituitary-adrenal axis was evaluated in nine normal subjects. The subjects underwent standard oral metyrapone tests (750 mg every four h, for six doses) while taking no medications and while receiving oral Cypro (4 mg every six h). Cypro administration caused a significant reduction in: 1) baseline 17-hydroxycorticosteroid (17-OH) excretion (-31 +/- 7.2%), 2) the increase above baseline in 24 h 17-OH excretion on the day after metyrapone (-32 +/- 4.9%), 3) serum 11-deoxycortisol concentrations 8 h (-45 +/- 5.5%) and 24 h (-18 +/- 3.6%) after the first dose of metyrapone, and 4) plasma ACTH concentration 8 h (-32 +/- 8.2%) and 24 h (-22 +/- 8.0%) after the first dose of metyrapone. When compared with the control test, Cypro administration did not alter: 1) baseline 17-ketosteroid secretion, 2) the fall in serum cortisol concentration while taking metyrapone, 3) the serum free metyrapone concentration, 4) cortisol metabolism, 5) the adrenal response to ACTH, and 6) assay methods for measurement of serum and urinary corticosteroids. Our data suggest that Cypro can reduce pituitary-adrenal responsiveness by reducing plasma ACTH concentrations. If Cypro acts as a serotonin antagonist, our data lends support to the idea that serotoninergic mechanisms are important in the control of pituitary ACTH secretion in normal human subjects.