Hyperbaric oxygen (HBO) therapy is a recognised treatment for problem wounds, but it has been suggested that the therapy may cause oxidative stress in blood (Narkowicz et al. 1993). Platelets are able to generate reactive oxygen species (Wachowicz et al. 2002). Therefore, to elucidate whether HBO therapy causes oxidative stress in platelets, platelets were isolated from whole horse blood (blood supplied by TCS Biosciences, Buckingham), and exposed to (A) 100% oxygen at 2.2 atmospheres, (B) 100% oxygen under normobaric conditions, or (C) air under normobaric conditions for 90 minutes. Samples were analysed pre- and post-treatment, and 3 h and 24 h post-treatment. All data are for n = 6 replicates. Pre-treatment platelet count was 5.07 x 107 ± 0.94 cells ml-1 (mean ± S.E.). There was no treatment-dependent effect on cellular ATP (Kruskal Wallis, P > 0.05), although all treatments showed a transient rise in ATP at 3 h post-treatment (e.g. in control platelets at 3 h, median ATP was 11.1 nmol/mg protein; range, 8.60 − 16.4, compared to background levels of 8.75 nmol/mg protein; range,6.59 − 10; P = 0.045), which was consistent with continuing platelet viability. There was an expected cumulative increase in lactate content of the medium after 3 h in all 3 treatments (Kruskal Wallis, P = 4.33×10-8), but no differences between treatments over time. No difference in platelet superoxide dismutase (SOD) activity was detected between treatments (Kruskal Wallis, P > 0.05), although all treatments showed a rise in SOD after 24 h in culture (e.g. in control platelets at 24 h, median SOD was 0.24 units/mg protein; range, 0.20 − 0.30, compared to initial background levels of 0.18 units/mg protein; range, 0.12 − 0.21; P = 0.01). Catalase platelet levels did not change over time, nor between treatments, and remained at around 12 units/mg protein. These initial data suggest that HBO therapy has no detrimental effect on platelet survival and does not cause overt oxidative stress. Analysis of reduced glutathione levels, and lactate dehydrogenase from platelets are in progress.