The incorporation of labelled thymidine (dT) and iododeoxyuridine (IdU) into DNA was studied with tissue culture cells and with normal mouse cells in vitro. The rates of incorporation and the ratio dT/IdU incorporation both varied from one type of cell to another and from one suspending medium to another. Despite the known complexity of the regulation of DNA synthesis, the data for incorporation of exogenous dT and IdU could be fitted reasonably well by a model for single-step enzymic process. Deviations from the theoretical predictions were minimal in the presence of fluorodeoxyuridine.