Different species of animals (warm-blooded, poikilotherms and coldblooded) were exposed to oxygen high pressure (OHP) (1–4 ata) for 2–8 h. OHP revealed (a) a different vulnerability in various species; (b) an inverse correlation between basal oxygen consumption and their vulnerability; (c) the critical point of the toxic effect of OHP in normothermia was an exposition time of 2 h with 2 ata OHP; (d) morphological changes of lung, CNS and vessels – intense folding of alveolar septa with cell hyperplasia and hypertrophy; (e) electron-microscopical demonstration of damaged endothelial cells with total or partial occlusion of the capill