1. Alkaline phosphatase, sucrase, Na + , K + -ATPase and Mg 2+ -ATPase specific activities of crude membrane fractions, prepared from duodenal, jejunal, ileal and colonic mucosa, have been estimated in three types of hypertensive rats: the spontaneously hypertensive rat (SHR), the DOCA-saline-treated rat and the renovascular rat (Goldblatt one-kidney, one-clip rat; 1K-1C). Alkaline phosphatase and sucrase specific activities have also been measured in purified jejunal brush-border membranes. 2. When compared with its normotensive age-matched control (WKY rat), the SHR has a lower activity of alkaline phosphatase in duodenal and jejunal crude membrane fractions, whereas a higher activity in colonic Na + , K + -ATPase is recorded. In purified jejunal brush-border membranes, lower alkaline phosphatase activity and higher sucrase activity were found. These differences occur in the young prehypertensive SHR as well as in the adult animal. 3. In the DOCA-treated rat, the only significant alteration in crude membrane fractions is a decreased Mg 2+ -ATPase activity at all regions of intestinal mucosa. In purified jejunal brush-border membranes both alkaline phosphatase and sucrase activities are increased at 4 or 7 weeks but especially at 13 weeks of hypertension. 4. In the 1K-1C rat, no significant modification appears in crude membrane fractions or in purified jejunal brush-border membranes, but a decrease in alkaline phosphatase and in sucrase activities is probable afer 13 weeks of hypertension. 5. Since alterations of the intestinal enzymes are different in the three types of hypertensive rats it is concluded that the changes are not secondary to the hypertension condition. In the SHR, these alterations are present in the young prehypertensive animal. Abnormalities of intestinal function may be implicated in the origin of genetic hypertension. 6. All recorded alterations represent 25-50% variations from control values. They indicate disturbances of intestinal ion transport in the three types of hypertensive animals.