The flooding and anoxia tolerance of three barley cultivars (Hordeum vulgare L. cv. Kustaa, Hankki ja-673 and Pokko), beaked sedge (Carex rostrata Stokes), rice (Oryza sativa L. cv. FR13A) and yellow flag (Iris pseudacorus L.) have been assessed by determining the length of anoxia tolerance and the productivity of the barley cultivars under flooding. Other physiological parameters related to flooding and anoxia tolerance were also determined, and they included alcohol dehydrogenase activity and its enzyme kinetic parameters KM and Vmax (for reaction acetaldehyde to ethanol) in the barley cultivars and beaked sedge, production of ethanol, CO2 and some organic acids in seedlings of the barley cultivars under anoxia, and superoxide dismutase activity in barley, rice and yellow flag under hypoxia and anoxia. A study of the root anatomy of barley and beaked sedge was also conducted. The anoxia tolerance experiments revealed differences in the three barley cultivars and the flooding tolerance experiment gave very similar results. Thus, Kustaa proved to be the most anoxia tolerant and most productive under flooding, Hankki ja-673 being intermediate and Pokko the least anoxia tolerant and the least productive during flooded conditions. Simultaneous measurements of ethanol, carbon dioxide and organic acid production during anoxia together with determination of dry weight loss during the period of oxygen deprivation showed that the rate of ethanolic fermentation was significantly faster in the more intolerant cultivars. Also, the more anoxia intolerant cultivars lost more of their dry weight during the anoxic incubation than the most tolerant cultivar. These results were in agreement with the metabolic theory of flooding tolerance. Superoxide dismutase activity measurements in the barley cultivars as well as in rice under hypoxia did not reveal any changes in the activity with the onset of oxygen deprivation. Neither did incubation under total anoxia increase SOD activity. In yellow flag rhizomes, which are very flood and anoxia tolerant, large increases in SOD activity took place during and after oxygen deprivation indicating greater protection against subsequent oxidative damage. Injuries of this nature have been noticed earlier in flood intolerant species. The anatomical study of the roots and rhizomes of beaked sedge and roots of barley cultivars revealed the larger percentage of aerenchyma in beaked sedge.