The levels of S-adenosylethionine (AdoEt) and of S-adenosylmethionine (AdoMet) in the livers of weanling male rats and male and female hamsters fed ethionine for 1-6 weeks were determined. Ethionine was fed at levels of 0, 0.1, and 0.3% in the diet, and the animals were sacrificed after 0, 1, 3 and 6 weeks of treatment. In both species the hepatic contents of AdoEt were dependent upon the level of ethionine in the diet. For the 6-week experimental period the hepatic levels of AdoEt average 81 microgram/g liver in male hamsters fed 0.1% ethionine in the diet and 160 microgram/g in those fed 0.3% ethionine; the corresponding AdoEt levels in female hamsters were 104 and 191 microgram/g liver, respectively. No marked shifts in hepatic AdoEt levels were seen in either male or female hamsters although a gradual rise in hepatic AdoEt from 145 to 233 microgram/g was noted in the female hamsters receiving 0.3% ethionine in the diet for 1-6 weeks. AdoEt levels in the livers of rats fed 0.3% ethionine were quite variable with values of 123, 305 and 127 microgram/g liver noted at weeks 1, 3 and 6 respectively. In rats fed the 0.1% ethionine diet the liver AdoEt levels dropped from 103 to 61 microgram/g from weeks 1 to 6, In animals fed the ethionine-free diet, the hepatic contents of AdoMet were relatively constant throughout the 6-week experimental period, with average values of 25, 17, and 29 microgram/g liver respectively in the male rats, male hamsters and female hamsters. Chronic ethionine administration always suppressed hepatic AdoMet levels. This suppression was generally greater in animals fed the 0.1% ethionine than in those fed the 0.3% ethionine diet. Thus, the average hepatic AdoMet level in rats, male hamsters and female hamsters receiving the 0.1% ethionine diet for 3-6 weeks were 32, 18, and 45% respectively, of the corresponding AdoMet levels in control animals: however, the corresponding AdoMet levels in animals receiving the 0.3% ethionine diet were 66, 42, and 62% of the respective control values. Feeding 0.1% ethionine to male hamsters led to exceedingly low levels of liver AdoMet (1.4-2.9 microgram/g). No direct correlations could be made between the effects of ethionine feeding on the hepatic AdoEt and AdoMet levels in rats and hamsters and the previously reported differences in carcinogenicity by ethionine in these species.