Sulphobromophthalein (BSP) retention and liver glutathione levels were studied in uncomplicated hyperthyroid patients. BSP retention was increased in 52.5% of the subjects at admission (n = 40) and in 28% of the cases after 3 months of propylthiouracil treatment (300-400 mg/day) (n = 25). Hepatic levels of glutathione were measured in six patients and significant inversed power correlation with BSP retention was observed (r = 0.968, P < 0.001). These data support the contention that the alteration of the BSP retention observed in hyperthyroidism could be due to a decreased availability of hepatic glutathione for conjugation.