The effect of 8-hydroxyquinoline, a rapid inhibitor of RNA synthesis, was followed on the activity of a number of enzymes in cultures of the fission yeast Schizosaccharomyces pombe. Two types of effect were found. In the first the activity continued to rise for a period and then remained constant. This occurred with alkaline phosphatase, basal and derepressed acid phosphatase, hexokinase, and derepressed sucrase and maltase at low cell density. It is consistent with control being exercised by an unstable mRNA or by an unstable stimulator of translocation. In the second the activity increased above the control values for several hours. This occurred with basal sucrase and maltase, and suggests a stable mRNA and an unstable inhibitor of translation. The extent of 'superproduction' of sucrase varied with cell density and with growth medium and this may be due to differences in the degree of translational inhibition. The possiblilty of a stable mRNA has interesting implications for the control of enzyme synthesis through the cell cycle.