BHA was administered to Wistar rats at a dose level of 2% in a powdered diet for periods of 1, 2 and 4 wk. After 1 wk epithelial damage, mild hyperplasia and hyperkeratosis of the forestomach mucosa was observed. The hyperplasia and hyperkeratosis showed progression at wk 2 and 4 whereas other epithelial defects regressed. The lesions were most pronounced in the vicinity of the limiting ridge. A further 4 wk of feeding without BHA resulted in a complete regression of epithelial defects, although the hyperplastic changes were still apparent. Other rats were given 1 g BHA/kg body weight/day by gastric intubation in arachis oil for 1, 2, 4, 8, 16 or 32 days. Increased mitotic activity was observed after 1 day and mild hyperplasia after the second intubation, but inflammatory response and superficial defects were not prominent and the hyperplasia of the squamous epithelium did not appear to result from initial damage and subsequent hyper-regenerative activity. A gradual regression of the hyperplastic changes occurred after eight daily intubations. The lesions were found in the apex of the forestomach remote from the limiting ridge. It is concluded that BHA incorporated in powdered diet or given in arachis oil by oral intubation causes lesions in the rat forestomach similar to that reported for BHA given in a pelleted diet (Ito et al. J. natn. Cancer Inst. 1983, 70, 343; idem, Gann 1983, 74, 459). The hyperplastic changes in the mucosa occur rapidly and their localization is dependent on the mode of application. Following withdrawal of the BHA there was almost complete regression of the lesion, only a residual mild hyperplasia remaining after 4 wk.