The reversibility of vernalisation in Petkus winter rye has been studied in detail, by using alternating periods, during germination, of low and high temperature of varying durations. The degree of devernalisation by exposure to subsequent high temperature varies with the initial duration of vernalisation and becomes progressively less as vernalisation proceeds. A quantitative method of studying these effects is described, and a revised schema of the dynamic aspects of vernalisation put forward.