Changes in the structuredness,i.e. average microviscosity, of cytoplasm in yeast cells progressing from the stationary to the logarithmic phase of growth were investigated. The concomitant variations in the first order rate constant for the enzymatic hydrolysis of fluorescein diacetatein vivo were found to be inversely related to variations in the structure lness of cytoplasm. The hypothesis that the structuredness of cytoplasm participates in the control and regulation of rates of enzyme reactions is put forward.