Résumé : The present study was made to investigate the possible effects of dexamethasone and secobarbital on the pituitary responses of prolactin (PRL) and TSH to TRH. enantiomeric forms of the steroids were detritiated faster than the 21R [21 3H] forms. (1 mg) and secobarbital (100 mg) were administered separately or together at 11 p.m. An intravenous injection of 200 μg TRH was given at 9 a.m. the following morning. PRL control levels and release were not modified by prior administration of the drugs. TSH control levels remained unchanged but TSH release was significantly inhibited by the combined pretreatment; dexamethasone and secobarbital given alone had no effect. The present study supports the view of a direct synergistic effect of both drugs on the pituitary thyrotropes, diminishing their responsiveness to TRH.