Cyclic AMP first decreased in wahed platelets after the addition of 0.1 IU thrombin/ml and then increased progressively. Platelets pre-incubated with acetylsalicylic acid showed only a decrease of cyclic AMP upon addition of thrombin. Furthermore, two patients with defective prostaglandin synthesis had always reduced cyclic AMP whether their platelets were pre-incubated with acetylsalicylic acid or not. Thus, if prostaglandin synthesis is inhibited the level of cyclic AMP decreased and did not increase with time.