The effects of short- and long-term ethanol administration on the hepatic content of free proline and on the activity of hepatic enzymes that catalyze the formation and degradation of proline were determined in the rat. The short-term oral administration of ethanol in a dose of 5.5 gm/kg body weight resulted in no changes in hepatic free proline content or in hepatic proline oxidase activity. By contrast, the feeding of ethanol for a period of 1 month resulted in an increase in the total hepatic content of free proline. The hepatic activity of proline exidase was also increased by long-term ethanol feeding while the activities of arginase, ornithine aminotransferase, delta 1-pyrroline-5-carboxylate reductase, delta 1-pyrroline-5-dehydrogenase, and glutamate dehydrogenase remained unchanged. The increase in the hepatic pool of free proline in association with an increase in proline oxidase activity suggests that long-term ethanol administration results in an increased turnover of proline in the liver, in which the increase in synthesis is greater than the increase in degradation. An effect of long-term ethanol feeding in increasing proline degradation mya be a cause for the increased oxygen consumption and urea production found in the liver after long-term ethanol ingestion.