Effects of balneotherapy on platelet glutathione metabolism were investigated in 12 type II (non-insulin-dependent) diabetic patients. Levels of the reduced form of glutathione (GSH) on admission were well correlated with those of fasting plasma glucose (FPG; r = 0.692, P < 0.02). After 4 weeks of balneotherapy, the mean level of GSH showed no changes; however, in well-controlled patients (FPG < 150 mg/dl), the level increased (P < 0.01) and in poorly controlled patients (FPG > 150 mg/dl), the value decreased (P < 0.05). There was a negative correlation between glutathione peroxidase (GPX) activities and the levels of FPG (r = -0.430, P < 0.05). After balneotherapy, the activity increased in 5 patients, decreased in 3 patients and showed no changes (alteration within +/- 3%) in all the other patients. From these findings in diabetic patients we concluded: (1) platelet GSH synthesis appeared to be induced in response to oxidative stress; (2) lowered GPX activities indicated that the antioxidative defense system was impaired; and (3) platelet glutathione metabolism was partially improved by 4 weeks balneotherapy, an effect thought to be dependent on the control status of plasma glucose levels. It is suggested that balneotherapy is beneficial for patients whose platelet antioxidative defense system is damaged, such as those with diabetes mellitus and coronary heart disease.