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The oxygen equilibrium properties of blood and of solutions of haemoglobin from Xenopus laevis are reported. At pH 7·6 the oxygen affinity of the blood, expressed as half saturation oxygen tension (P00) amounts to 27·0 mm and 13·7 mmHg (3·60 and 1·83 kPa) when measured at 25 and ro °C, while the Bohr factor ( lllog P 50/ ll pH) was - o · 40, and the Hill's coop era tivi ty coefficient, n, averaged 2·r. These data reflect an overall heat of oxygenation, llH, of -7·9 kcal. mol-1, which decreased to - 6·3 kcal. mol-1 when the live animals were acclimated to each measuring temperature. Xenopus blood showed a high 02 capacity ( r 5 vol.%) compared to that of other amphibians. Acclimation to water of increased salinity ( 12%0), and aestivation, raised blood 02 affinity; at 25 °C and pH 7·6, P50 decreased to 21·1 and 25·2 mmHg (2·81 and 3·36 kPa), respectively. These changes were concomitant with increases in the blood levels of urea. In contrast to Na Cl and ATP, urea increased 02 affinity of the purified haemoglobin, suggesting that oxygenationlinked binding to haemoglobin is involved in the modulations of the blood 02 affinity during aestivation and acclimation to salt water.