A significant correlation was found between the inhibition produced by 1% halothane with nitrous oxide and oxygen on platelet aggregation in vitro and the increase in bleeding time during anaesthesia with halothane, nitrous oxide and oxygen in 10 patients. It is suggested that halothane in nitrous oxide with oxygen inhibits platelet aggregation in vivo and in vitro. The inhibition is not seen when platelet aggregation is studied in platelet-rich plasma from anaesthetized patients because the agents evaporate during preparation of platelet-rich plasma and during analysis in the aggregometer.