The relative iron (II) tolerance of a range of wetland plants was determined and compared with some species characteristic of well drained soils. A wide range of tolerance occurred amongst the wetland species but they were generally more tolerant than those from well drained soils. No correlation was found between iron (II) tolerance and the amount of air space {% v/v) (aerenchyma) in the roots of these species. There was a significant negative correlation between air space and iron uptake by roots. This may have been caused by iron (II) oxidation in the rhizosphere resulting in decreased availability. There was evidence that differential iron (II) tolerance of excised root tips was maintained under aerobic and anaerobic conditions. It was thus suggested that iron (II) tolerance may not be dependent on iron exclusion or oxidation of iron (II) by oxygen diffusing through the aerenchyma. Levels of malic and citric acids in roots were altered by iron (II) sulphate, but the absolute levels and changes in levels had no correlation with the iron (II) tolerance, of the species. Peroxidase and catalase activities in root tips of plants gown in drained and flooded sand culture were measured and considered in relation to the oxidising power of roots. Activity was detected in all species examined but was generally I unaffected by flooding. Evidence from the literature suggested that these enzymes of peroxide metabolism are unlikely to be active in flooded roots and so could not mediate their oxidising power. The structure of root aerenchyma had great variability between species. The Cyperaceae had the most complex and well organised structure. Growth under flooded conditions increased air space in most species, but there were exceptions. In Eriophrum angustifolium and E. vaginatum air space was high under drained conditions and was not increased by flooding. In Filipendula ulmaria the small amount of air space was not increased by flooding. Low nutrient levels increased air space production in Nardus stricta. The function of aerenchyma and the influence of environmental factors on its production are discussed.