Abstract An outline is given of a concept of differentiation and oncogenesis which is based on the existence of cellular biochemical periodicities. It is proposed that the properties of a cell are largely determined by the pattern of rhythmic variations in constituent levels resulting from the operation of numerous metabolic control circuits. It is suggested that cellular differentiation and oncogenesis are processes involving alterations in the absolute and relative values of the dynamic parameters governing the periodicities and hence changes in the pattern of cellular rhythms. Accordingly, the actions of oncogenic agents are interpreted in terms of temporal disturbances. Fertilisation is considered to be a perturbation initiating rhythmic response in the control circuits while organogenesis and aging are believed to reflect the approach of cells (and the organism as a whole) toward a new aperiodic state as a result of dampening processes. Such views indicate that malignant transformations can occur spontaneously. Some experimental evidence, indicating periodic isozyme pattern changes in cells, is presented in support of the concept.