1. The rate of mucosal to serosal water movement was measured in vitro in non-everted midgut segments in Atlantic salmon (Salmo salar L.) during parr-smolt transformation (February-July) and following transfer of smolts to seawater in May. 2. The rate increased significantly during smolting from 5.61 microliters/cm2/hr in the parr (February) to 11.03 microliters/cm2/hr in smolts in May. 3. Measured at intervals over a period of 20 days in seawater, the rate of water transport was not significantly different from that found in the freshwater-adapted smolts (11.20 microliters/cm2/hr). 4. Intestinal water transport is sodium-linked and inhibited by ouabain but is not stimulated by cortisol.