During a 10-day culture ageing, cells of the wild-type Saccharomyces cerevisiae strain JC 482 retain their viability, while mitochondrial function and morphology change. Cell routine and uncoupled respiration rates increase to a maximum on day 4 and then decline to near zero. The decline, which occurs also in mitochondria isolated from cells of different age, is not due to increasing proportion of petites. Rhodamine 123 fluorescence intensity reporting on mitochondrial membrane potential appears to drop slightly for 4 days and then more sharply at the time when respiration rate also decreases. The MitoTracker Green fluorescent signal related to the mitochondrial content per cell also decreases. The branched tubular mitochondrial network of 1-day-old cells dissolves into short fragments; during the first 4 days, this fragmentation is associated with increasing function of mitochondria, while later on, it accompanies functional decline, which is also indicated by the decreasing ratio of Rhodamine 123 fluorescence to MitoTracker Green fluorescence. As shown by cell counting, microscopy and flow cytometry, the cell size distribution in the population broadens, and the population thus becomes more heterogeneous. The changes in respiration rate, mitochondrial membrane potential, mass and structure point to changes in the mitochondrial status during ageing.