Active transport of ascorbic acid in ileum is mediated by a carrier mechanism at the brush border membrane. This mechanism may show compensatory changes in activity in response to alterations of dietary ascorbic acid content. The unidirectional influx of ascorbic acid across the brush border into epithelial cells of guinea pig ileum was determined in vitro. Influx was significantly reduced in scorbutic animals and following 14 or 28 days of high doses (5 or 25 times normal) of ascorbic acid. The transport rate was reduced by intramuscular administration of ascorbic acid, suggesting that the transport mechanism may respond to circulating levels of the vitamin.