Measurements of the rate of fruit drop and the hormone content of the seeds, made at weekly intervals on three varieties of apple, have shown that periods of very active hormone production are invariably associated with periods of low fruit drop. On the basis of these and other observations, an hypothesis is proposed to account for the periodicity of fruit drop. It is suggested that, throughout its development, the fruit possesses an inherent tendency to separate from the spur but that, at certain periods, it is prevented from doing so by hormonal stimuli emanating from the developing seeds. Two such stimuli have been demonstrated, at 3–4 weeks and 7–10 weeks after petal-fall respectively, both of which are associated with particular developmental stages of the endosperm. In addition the existence of a third, as yet undetected stimulus at the time of fertilization, is postulated.