An attempt was made to elucidate those factors associated with the seed coat which are responsible for maintaining the dormancy of the charlock seed. The rates of water uptake by the seed and by the embryo were measured; the permeability of the covering layers to water was much the same as that of parenchymatous cells. The action of certain dormancy-breaking treatments, viz. embryo excision, exposure to concentrated sulphuric acid, alternating temperatures, and gibberellic acid, were therefore examined.From experiments with excised embryos and seeds exposed to concentrated sulphuric acid, it appeared that the loss of dormancy following the removal of the seed coat resulted from an increase in oxygen supply to the embryo. The germination of a small proportion of seeds at low temperature may also be due to an increase in the oxygen concentration within the seed. The dormancy-breaking effect of gibberellic acid, however, is unlikely to be due to changes in the permeability of the seed coat to oxygen. Estimates of the rate of oxygen uptake and growth of seeds treated with gibberellic acid at various external oxygen concentrations show that the resistance of the covering layers to diffusion of oxygen retards, but does not prevent, the initiation of growth.It would appear that dormancy is maintained in charlock seeds by the action of a specific growth-inhibiting substance which is produced at low oxygen concentration in the interior of the seed and which diffuses to the meristems.