Twelve otherwise healthy male volunteers scheduled for arthroscopy of the knee were studied. The influencein vivoof nitrous oxide (N2O)per seand the addition of a halogenated volatile anaesthetic (halothane or isoflurane) on ADP–induced platelet aggregation and release of beta–thromboglobulin into plasma was evaluated. All measurements were made before surgery. We found that N2O increased platelet aggregation. Adding a halogenated anaesthetic reversed the relative hyperaggregation induced by NaO. The concentrations in plasma of the platelet release product beta thromboglobulin were not influenced by the anaesthet