Hexachlorobenzene (HCB) was fed to male and female F344 rats as 0.02% of the diet for 15 weeks. Females developed a massive porphyria, due to depression of uroporphyrinogen decarboxylase activity, whereas males did not. Although hepatic non-haem iron levels in control females were 3-5 times greater than males (iron is implicated in the pathogenesis of this condition) preloading the latter with iron did not increase their susceptibility. After 90 weeks of HCB treatment 100% of surviving females had multiple liver tumours which were strongly gamma-glutamyl transpeptidase (GGT) positive and histologically classified as neoplastic nodules or hepatocellular carcinomas. In contrast, only 16% of males developed tumours which were smaller and fewer in number per liver than those in females. Accumulation of porphyrins was still significantly less in males than females although no uroporphyrinogen decarboxylase activity was detected in treated livers of either sex. No differences in porphyrin levels or enzyme activity were found between tumours and surrounding tissue showing that tumours did not revert to a non-porphyric state. The sex difference in tumour response could not be explained by differences in hepatic HCB concentrations. Non-haem iron concentrations of livers fell after HCB treatment for 90 weeks in both sexes and were even lower in tumours. These studies demonstrate that not only are female rats far more sensitive than males to the porphyrinogenic effects of HCB but also to the hepatocarcinogenic actions, suggesting a link between these two manifestations of toxicity that may also apply to other polyhalogenated aromatics.