Plasma beta-endorphin, adrenocorticotropin (ACTH) and blood polyamine (spermidine and spermine) concentrations were evaluated in healthy adult male athletes undergoing hyperbaric oxygen exposure for 10 days (2.8 atm, 100% O2, 60 min daily). In the "acute phase", corresponding to the first day of treatment, and in the "acute in the chronic phase", corresponding to the values obtained on the 5th and 10th days after 60 min of hyperbaric O2, both ACTH and beta-endorphin levels increased significantly, whereas no variations were observed for polyamine concentrations. In the "chronic phase", corresponding to the basal values of the 5th and 10th days of treatment, we found a different pattern. In fact, the concentration of polyamines showed a remarkable enhancement, while ACTH and beta-endorphin levels remained unchanged. No significant variations were observed during hyperbarism with air. These results demonstrate different modifications of polyamines and beta-endorphin and ACTH in subjects submitted to hyperbaric oxygen exposure.