In order to study the effect of prostaglandins on TSH release in the human, Acetylsalicylic Acid was given to 9 normal subjects at doses necessary to obtain a salicylate level of 20 mg/dl. TRH, 400 mug was injected prior to and after the medication. Thyroid hormone parameters were measured at time 0, and TSH and PRL at time 10, 20, 30, 45 minutes after the injection. The free fraction of thyroid hormones remained constant during treatment but a significant decrease of TSH concentration (30%) was noted after TRH injection on Acetylsalicylic Acid treatment whereas no changes were noted for PRL. As noted in the animal, this study indicates that in the human, prostaglandins potentiate the effect of TRH on TSH liberation by the pituitary where it has no effect on PRL.