During the cell cycles of synchronous cultures of several different yeasts and protozoa, oscillations in the rate of respiration and of total cellular protein content have been demonstrated. Energy supply (from mitochondrial oxidative phosphorlation) and energy demand (biosynthetic reactions, especially protein accumulation) are closely coupled oscillating systems. Phase correspondence between O2consumption rates, intracellular ADP pool size, and total cellular protein indicates that it is energetic demand that determines mitochondrial activity (respiratory controlin vivo). The dynamics of the coupled oscillators indicate that the rate‐determining control circuit operates on a time scale expected of epigenetic reactions (transcription and translation). Thus the energy‐yielding reactions are enslaved to the slower time constants of biosynthesis. Temperature compensation indicates a timing function, and a common phase reference point makes subcycles commensurate with the cell cycle. It is suggested that cell cycle timing, by counting subcycles in growing cells, may become dominated by circadian control in slowly growing natural populations, and that the same subcycles may be used for circadian timekeeping. The discontinuous nature of growth suggests extensive and rapid turnover of macromolecular cell components organised in alternating temporal compartments of biosynthetic and degradative processes.