Some physiological effects of freezing and flooding were studied on winter barley (Hordeum vulgare L.). The effects of freezing on plants grown under field conditions were determined by comparing plants exposed to naturally-occurring freezing temperatures with plants protected from such temperatures by thermostatically-controlled soil heating cables. The results from field trials in 1982/83 and 1983/84 showed the effects of freezing to include: (i) A reduction in the establishment of late-sown plants due to soil heaving. (ii) A retardation of plant and apical development. (iii) Occasional transient reductions in above ground dry matter and leaf area. (iv) Variable effects upon certain yield components. Attributes not greatly affected by such temperatures included cold hardiness and yield on a per plant basis. Controlled environment studies showed that non-lethal freezing temperatures ranging from -4 to -10oC significantly reduced shoot photosynthesis and dark respiration, the greater reductions being in photosynthesis. Factors involved in these reductions include damage to leaves and roots, neither of which were critical for plant survival. When a conductivity method was used to estimate the damage to different plant parts resulting from extracellular freezing, leaf laminae were found to be more freezing tolerant than roots, and young laminae were more tolerant than older laminae. Some effects of flooding were also studied using controlled environments, and cultivar differences in flooding tolerance were found. This tolerance was investigated by comparing the response to flooding of a tolerant cultivar (Athene) with a sensitive one (Maris Otter). Flooding sensitivity was shown as wilting in the leaves of Maris Otter, the onset of wilting being associated with the leakage of electrolytes and accumulation of ethanol in the leaf laminae. The results of exogenous application of ethanol to leaves, suggest the basis of this `tolerance' is ethanol exclusion. The production of aerenchyma was not associated with flooding tolerance.