OBJECTIVES: To investigate the effect of hyperbaric oxygen (HBO) on platelet physiology. DESIGN AND METHODS: Human platelets were exposed to HBO (97.7% O(2), balance CO(2) at 2.2 ata) or control (CON; 5% CO(2), balance air at 1 ata) for 90 min, and analyzed for aggregation, protein release, ()NO production, and activation. RESULTS: HBO induced 29.8+/-3.0% of platelets to aggregate compared with CON (5.5+/-0.9%). Proteins observed to be released in greater abundance from HBO- compared with CON-treated platelets included 14-3-3 zeta and alpha-2-macroglobulin. Release of ()NO by platelets was unaffected following exposure to HBO, as was platelet activation as measured by surface expression of PECAM-1, CD62P and the activated form of alpha(IIB)beta(IIIa). CONCLUSIONS: Exposure to HBO induces both platelet aggregation and protein release. Further study will better define the precise mechanisms and effects of HBO on platelet activation.