The synthesis of the adenosyl-moiety of S-adenosylethionine from glycine was studied in normal rats and rats fed a diet containing 0.30% DL-ethionine for three weeks, using a L-ethionine probe. The liver of rats, pretreated by DL-ethionine feeding, incorporated--after injection of 20 mg L-ethionine/100 g body wt.--six times more C-14 from glycine-2-14C into the adenosyl-moiety of SAE than normal rats. Further increasing of doses up to 100 mg/100 g body wt. did not substantially change the incorporation in pre-exposed rats while in normal rats an almost complete inhibition of the incorporation was observed. The liver cells exposed to a chronic effect of DL-ethionine are able, after higher demands for ATP, to increase the ATP generation more effectively than normal cells. This adaptation to the toxic effect of ethionine represents one of the first alterations of the metabolic functions of liver parenchyme in the course of hepatic DL-ethionine cancerogenesis.