The time course of incorporation of radioactivity into liver phosphatidylethanolamine and its precursors was studied following intraportal injection of [1,2-14C]ethanolamine into rats that had been fasted for 24 h and into nonfasted rats. Marked diminution in the labelling of liver phosphatidylethanolamine and CDP-ethanolamine was present in the fasted groups while the radioactivity contained in the ethanolamine and phosphorylethanolamine pools of these livers was increased. The livers of the fasted rats contained significantly higher levels of ethanolamine, phosphorylethanolamine and CDP-ethanolamine while choline and phosphorylcholine were decreased. These changes are identical to those which we have described earlier in the livers of nonfasted rats fed a choline deficient diet. In a further experiment, the duration of the fasting period necessary to bring about these changes was studied. Incorporation of radioactivity into liver phosphatidylethanolamine at 2.5 min following intraportal injection of [1,2-14]ethanolamine declined progressively up to 7.5 h of fasting and did not decrease further after that time. There was a concomitant decrease in labelling of CDP-ethanolamine with retention of radioactivity in ethanolamine and phosphorylethanolamine. These results show that the liver metabolism of ethanolamine is altered quite early in the fasting period and suggest that it may be related in some way to the metabolic response to fasting.