The effect of a toxic dose of acetaminophen on hepatic glutathione turnover was studied in fed and fasted rats. Following the administration of 1 gm per kg of acetaminophen, the fractional rate of glutathione turnover increased from 0.19 to 0.28 hr-1 in fed rats and from 0.43 to 0.50 hr-1 in rats fasted for 48 hr. The increase in the fractional rate of turnover was proportionally much less than the decrease in hepatic glutathione concentration resulting from the toxic dose of acetaminophen. Thus, the estimated hepatic synthesis of glutathione decreased from 0.86 and 1.50 mumole per gm liver . hr to 0.59 and 0.53 mumole per gm . hr in fed and fasted rats, respectively. The excretion of acetaminophen-sulphate was significantly decreased in fasted rats. The fraction of administered acetaminophen metabolized to the mercapturic acid, however, was similar in both groups. Nevertheless, all fasted animals had massive centrilobular necrosis whereas virtually no necrosis was observed in fed animals. The administration of sulfate did not protect against liver injury in fasted rats. In view of the similar synthesis of glutathione in fed and fasted rats following a toxic dose of acetaminophen, an inability to synthesize adequate quantities of glutathione or a decreased availability of sulfate cannot account for the increased susceptibility to liver injury in the fasted state. However, a toxic dose of acetaminophen depletes the hepatic glutathione to a lower nadir before glutathione is repleted by synthesis, and a greater amount of tissue arylation occurs during this brief period of critically depleted glutathione.