Metabolic oscillations associated with the glycolytic pathway in intact cells and cell-free extracts of yeasts probably represent the first observation of prolonged self-sustained biochemical oscillations in vivo and in vitro. These oscillations have many characteristics similar to those of endogenous biorhythms which, others have postulated, might be driven by oscillations at the biochemical level. Among these characteristics are a stable frequency, a self-sustained nature, a susceptibility to phase shifting, fadeout, and reinitiation. The frequency is, however, temperature sensitive and high (approximately 1.8 minutes−1 in vivo and 0.17 minutes−1 in vitro).The metabolic oscillations, which represent a pulsing of the glycolytic flux, appear to arise by a mechanism involving an initial step of constant and specific velocity together with an activation of the enzyme phosphofructokinase by its product adenosine diphosphate. Other coupled reactions are also involved which may explain the appearance of "beats" and pulse-type oscillations, as well as the normal sinusoidal type of waveform. These oscillations are an interesting metabolic control phenomenon and allow the demonstration of apparent metabolic coupling between individual cells. They may also be useful as a model for a driving oscillation for endogenous biorthythms.