The rate of ageing of a batch of caryopses of Zea mays was accelerated by placing in a controlled environment of high temperature and humidity. Samples were withdrawn representing a sequence of increasing physiological ageing, and selected stages were subjected to ultrastructural and cytochemical investigation. Membrane aberrations increased with increasing age of the embryos. The damage was seen immediately upon imbibition, but there was much evidence of repair of this damage at later stages of germination. The existence and operation of mechanisms for the repair of such damage are discussed.