The duration of the phases of the cell cycle, for a population of cells, is generally studied with techniques which presuppose that the durations are either constant or change slowly relative to the duration of the whole cycle. We address the problems of measurement where cycle parameters change rapidly with increasing population age. This condition applies to a cohort of cells in a root tip and possibly to any culture that “ages”. We model the behaviour of a small population by using functions that give varying rates of progression through cycle phases as time passes. The status in the cycle of each cell at all times is determined by integration. Highly complex population behaviour is portrayed as a plot of trajectories, one for each cell. The ordinate gives the cycle status of the cell, the abscissa, time. The question is, can present methods, applied to such model population data, yield the original rate functions that prescribed the behaviour? Rigorous solution is not possible when the rates vary at will. The difficulty is that rate variation continually modifies asynchrony, or any other cell age distribution. This deprives most measurements, e.g. rate of accumulation of mitoses, of ready interpretation. As the number of detectable cycle phases increases beyond the present four, however, the ability to solve for the functions increases accordingly.