Temporal variations in the expression of the ras oncogene, and its protein product, were investigated during hexamethylene bisacetamide (HMBA)-induced differentiation of murine erythroleukaemic (MEL) cells. We highlight the fact that when comparisons were made between untreated, proliferating cells and HMBA-treated, differentiating cells using only one time-point, differences, both for the expression of the gene and the protein, were in most cases insignificant; standard deviations were high and the interpretation could be made that HMBA had little effect. Such interpretation fails to take account of the dynamic nature of the system, with single time-point studies giving incomplete information, which can be misleading. Multiple time analyses showed clearly that rhythmic patterns of expression were modulated by the differentiating agent. Time-dependent changes in the expression of mRNA specific to H- ras and N- ras, as well as in the expression of the Ras protein, when measured over periods of minutes or hours, were apparent. HMBA affected frequency and phasing of the rhythms. Regulation of the dynamics in this way may be crucial to the control of cell function and transformation.