The effect of 200 ppm hexachlorobenzene (HCB) in the diet on hepatic gamma glutamyl transpeptidase (GGT) levels was examined in F344 male and female rats. During early stages of feeding (4-29 weeks) some females had elevated GGT levels spread throughout the periportal regions, but all those examined at 29 weeks had a few GGT positive foci. The livers of males were not affected at this time. However, after 90 weeks of feeding, males also had elevated periportal GGT activity and a number of GGT positive foci. All females had lost the periportal activity but had developed GGT positive hepatocellular tumours. It therefore appeared that elevated nonfocal GGT levels are a poor indicator for HCB toxicity, but GGT positive foci may be a more reliable indicator of carcinogenic changes.