Uridine inhibits the incorporation of thymine in Escherichia coli 15T− cells but the duration of this inhibition is dependent upon the initial concentration of uridine and its rate of degradation. Uridine is rapidly degraded to uracil and the completion of this degradation coincides with the release of the inhibitory effect. However, uridine, by preventing the phosphorolysis of thymidine, allows the continued incorporation of the intact molecule at the initial high rate for as long as the concentration of undegraded uridine remains above a certain level. In resuspended 15T− cells, this rate is the same, irrespective of the presence or absence of uridine. The results presented here provide a simple method of controlling DNA synthesis, as represented by thymine incorporation, without the addition of inhibitors or other manipulations of the cell cultures.