Within 6 h of supplying ethylene to intact etiolated seedlings of Pisum sativum L. increasingly long profiles of rough and smooth endoplasmic reticulum (ER) appear in sections of epidermal and cortical cells from the hook region. By 24 h some profiles exceed 10 μm in length, more than 10 times the length of the longest commonly observed in control tissue. These profiles are not artifacts of the different preparations, for similar extended profiles occur in sections of pellets of rough ER which have been prepared from subapical internodes of treated seedlings by extraction and separation of the membranes on sucrose gradients. It is proposed that the changes in rough ER result from a 'stabilisation' of the membranes brought about by a reduced rate of phospholipid turnover in the presence of ethylene.