SUMMARY1. The absorption of NaCi and water was studied by intraluminal in vivo perfusion of the intestine of the yellow European eel (Anguilla anguilla) adapted to fresh water (FW), to sea water (SW), and to double strength SW (DSW).2. The net lumen to plasma NaCl transport from diluted SW perfusion fluids was independent of the NaCl concentration in the Na+ concentration range tested. The NaCl absorption (expressed as ,t-equiv/100 g . hr.) increased from FW (mean +S.E.): Na+ 166+ 17, Cl-205+ 24 to SW: Na+ 363 + 33, Cl-423 + 37, and again in DSW: Na+ 640 + 110, C1-676 + 149.3. The osmolality of the perfusion fluid which resulted in zero net water transport was higher than plasma osmolality by 73 + 3 m-osmole in FW, 126 + 5 m-osmole in SW, and 244 + 32 m-osmole in DSW (mean + s.E.). A fairly constant ratio between net NaCl transport and this osmolality difference prevailed.4. The general osmotic permeability to water in the serosa-mucosa direction (expressed as ,al./100g.hr.m-osmole) measured from experiments with impermeant solute increased from FW: 3-7 + 0-5 to SW: 7-2 + 1.0 (mean + S.E.).5. These results are compatible with the interpretation that the water flow occurring in the absence of a general transmural osmotic gradient, the 'solute-linked water flow', is linearly related both to net NaCl transport and to the osmotic permeability to water. The findings support the view that the 'solute-linked water flow' is, indeed, secondary to the salt movement and is due to osmotic force.