Early sowing of swedes increases yield but also increases the risk of bolting. The aim of the study was to investigate vernalisation, that is, low temperature flower induction, of swedes. In a series of experiments, plants of different ages and cultivars were given low temperature treatments of varying duration and temperature. Swedes were vernalised by temperatures of 110 and below, the optimum being around 50 to 6° for Wilhelmsburger and 30 to 6° for Doon Major. High temperatures following low temperature treatment and interruptions of treatment with periods at higher temperature were devernalising, reducing the numbers of plants flowering and decreasing the rate of flowering. Stem extension and response of stem growth to gibberellic acid were less affected by devernalisation than flowering. Plants grown at around 15P were found to have a juvenile stage of under 4 days, some cultivars having a shorter stage of 2 days or less. Low light intensities during vernalisation reduced the number of plants flowering but mature swedes could be vernalised in the absence of light. There was considerable variation in susceptibility to vernalisation in the cultivars used in the experiments. In order of decreasing susceptibility they were Pentland Harvester, Della, Wilhelmsburger and Marian, Harrietfield, Doon Major and Ruta Otofte. There was evidence of differences within cultivar variation, early and late flowering selected Wilhelmsburger lines differing more from the parent population in susceptibility to vernalisation than selected Doon Major lines. The longer the duration of low temperature the more plants flowered and the earlier they flowered. The normal site of vernalisation was found to be the growing point although axillary buds could be vernalised in the presence and absence of the growing point. There was no evidence of a translocatable flowering stimulus. Methods of selection and shortening the reproductive cycle are described.