When cucumber plants are chilled at 5 °C and 85 per cent, r.h. the leaves wilt rapidly and lose water. Chlorophyll is lost in the light, but not in the dark. If chilled leaves are placed in water, electrolytes leak out rapidly, the amount depending on the duration of chilling. There are marked reductions in the rates of respiration and photosynthesis; Q10 for respiration is 5.0 between 15 and 5 °C. Phospholipid levels decline, but more slowly than the rate of water loss. Leaves recover their initial fresh weight, rate of electrolyte leakage, and respiration and photosynthesis rates if the plants are returned to warm conditions within 1 to 2 d. When leaves are chilled at 100 per cent r.h. they do not lose water or phospholipids; electrolyte leakage is little more than in controls, but respiration and photosynthesis are reduced as at 85 per cent r.h. It is envisaged that the leaf cell membranes suffer a physical change at 5 °C which reduces the rates of respiration and photosynthesis, but allows the passage of water and electrolytes out of the cells; the water evaporates away but electrolytes become concentrated in the walls and can be leached out of the leaf. As water is lost from the leaf symptoms of dehydration appear. Phospholipid loss may be associated with enzyme action following the loss of compart-mentation.