Cyclooxygenase activity in human platelets reappears after ingestion of aspirin as a function of the platelet production rate. In different studies the activity reappeared without delay or with an interval of at least 48 h after stopping the drug. Because inhibition of megakaryocyte cyclooxygenase is the sole likely explanation for a delay we determined thromboxane B2 in human megakaryocytes obtained under local anaesthesia. We found that aspirin does not inhibit human megakaryocytes in vivo but does so in vitro. Therefore, it does not seem likely that there is actually a delay in platelet cyclooxygenase resurgence after aspirin intake. In order to suppress platelet cyclooxygenase constantly, aspirin should be given once or twice a day and not once or twice a week.