Leakage of electrolytes from discs of cucumber cotyledons is enhanced by floating them for 3 d on 10−3 M iodoacetate instead of water. The respiration of the discs is severely inhibited by iodoacetate and the total quantity of phosphatidyl choline, phosphatidyl ethanolamine, and phosphatidyl inositol falls to 77 per cent of the control level at 3 d in young cotyledons and 50 per cent in old cotyledons. 14C-acetate-labelling experiments provide evidence of phospholipid turnover; those phospholipids subject to the most rapid turnover tend to be the ones that disappear most rapidly from discs treated with iodoacetate.