Expression of the glycogenolytic action of glucagon in liver requires ATP for cAMP formation and for several subsequent phosphorylation reactions. To assess the extent to which ATP availability is rate-limiting to this hormonal action, responses to glucagon of intact liver and of liver with marked reductions in ATP content induced by ethionine was examined in female Wistar rats in vivo and in vitro. Compared to values in quick-frozen liver samples from control rats, basal hepatic ATP was 75% lower and cAMP, two fold higher in rats treated with ethionine. Activation of glycogen phosphorylase and inactivation of glycogen synthetase, phosphorylation reactions which require ATP and are initiated by cAMP, were also evident in basal liver samples from ethionine-treated rats. These hepatic alterations were associated with portal glucose and insulin levels which were significantly lower and portal glucagon levels which were four fold higher than values in controls. In ethionine-treated rats, glucose infusion decreased hepatic cAMP content and phosphorylase activity and increased synthetase activity. This and other observation suggested that the higher cAMP and the altered enzyme activities seen in vivo after ethionine administration were mediated by the hyperglucagonemia and/or by other endogenous glycogenolytic stimuli, and accordingly implied that liver remained responsive to such stimuli despite reduced ATP. Pharmacologic doses of exogenous glucagon clearly increased cAMP in vivo and in vitro in livers with decreased ATP. However, the lower ATP of liver exposed to ethionine was associated with a significantly blunted cAMP response to maximal glucagon stimulation. By contrast, alterations in phosphorylase and synthetase activities were not similarly blunted, suggesting that the smaller increases in cAMP seen in liver with reduced ATP content were adequate for the expression of these actions of the hormone. It is concluded that the actions of glucagon to increase cAMP and to activate phosphorylase and inactivate synthetase are not abolished by marked reductions in hepatic APT.