1. 1. The ion-exchange Chromatographie analysis of the red cell nucleotides of two water-snakes with different degrees of adaptation to water showed a ratio ATP/GTP of about 2.3 in the more aquatic species Helicops modestus and of about 0.7 in Liophis miliaris. 2. 2. The haemoglobin oxygen affinity assayed in the presence of ATP, GTP, DPG and IHP in haemolysate from both snakes were determined as a function of pH. The Bohr effect values were always found to be increased in comparison with the stripped haemolysate, becoming about 1.5 times higher in L. miliaris compared with H. modestus by the nucleotide treatment. 3. 3. The Hill coefficient calculated from the slope of the haemoglobin binding curves was consistent with an increase in the protein cooperativity in the presence of the nucleotides assayed. The very low nH values of about 1.2 in absence of the effectors became close to 2.0 in their presence. 4. 4. The data of nucleotide Chromatographie analysis were compared to those found in fishes with bimodal respiration. © 1984.783587589Antonini, Brunori, (1971) Haemoglobin and Myoglobin in their Reaction with Ligands, , North Holland, Amsterdam, Chap. 10Bartlett, Phosphate compounds in reptilian and avian red blood cellsdevelopmental changes (1978) Comp. Biochem. Physiol., 61 A, pp. 191-209Benesch, Benesch, The effect of organic phosphates from the human erythrocyte on the allosteric properties of haemoglobin (1967) Biochem. biophys. Res. Commun., 26, pp. 162-167Geoghegan, Poluhowich, The major erythrocyte organic phosphate of the American eel Angilla rostrata (1974) Comp. Biochem. Physiol., 49 B, pp. 281-290Koloustian, Poluhowich, The role of organic phosphate in modulating the oxygénation behavior of eel hemoglobin (1976) Comparative Biochemistry and Physiology Part A: Physiology, 53 A, pp. 245-248Matsuura, Ogo, Focesi, Multiplicity and immunological characterization of the haemoglobin components from Liophys miliaris and Helicops modestus (1984) Comp. Biochem. Physiol., 76 B, pp. 915-919Monod, Wyman, Changeaux, On the nature of allosteric transitions: a plausible model (1965) J. molec. Biol., 12, pp. 88-118Ogo, Abe, Focesi, Jr., Oxygen dissociation constant in hemoglobin of Helicops modestus and Liophys miliaris, two water snakes with different morphological adaptations to their aquatic environment (1979) Comp. Biochem. Physiol., 63 A, pp. 285-289Rapoport, Guest, Distribution of acid-soluble phosphorus in the blood cells of various vertebrates (1941) J. biol. Chem., 138, pp. 269-282Trader, Frieden, Dimerization and other chemical changes in amphibian hemoglobin during meth-amorphosis (1966) J. biol. Chem., 241, pp. 357-366Weber, Lykkeboe, Johansen, Biochemical aspects of the adaptation of hemoglobin-oxygen affinity of eels to hypoxia (1975) Life Sci., 17, pp. 1345-1350Wood, Johansen, Actaptation to hypoxia by increased HbO2 affinity and decreased red cell ATP concentration (1972) Nature, New Biol., 237, pp. 278-279Wood, Johansen, Organic phosphate metabolism in nucleated red cells: Influence of hypoxia on eel HbO2 affinity (1973) Neth. J. Sea Res., 7, pp. 328-338Wood, Johansen, Weber, Effects of ambient PO2 on hemoglobin-oxygen affinity and red cell ATP concentrations in a benthic fish, Pleuronectes platessa (1975) Respir. Physiol., 25, pp. 259-267