Abstract Experiments are described measuring the percental participation of applied exogenous TdR in the synthesis of DNA-thymine within the small intestine of mice during a given period of time. Mice were injected with 3HTdR which was partly diluted with nonradioactive TdR. The amounts of TdR were varied by a factor of 10 000. A combination of biochemical and autoradiographic measurements showed that only 0.1 – 1 per cent of the DNA-thymine synthetized during the experiment originates from applied exogenous TdR using activities of 3HTdR with high specific activity commonly mentioned in literature. Over a certain range this participation increases linearly with increasing quantities of applied TdR. The upper limit of the tracer region is ca 0.5 μg TdR/g animal weight. Relationships similar to that in the small intestine seem to exist in many other tissues too. The connection between the grain number/nucleus and the DNA-synthesis rate is discussed. Corresponding experiments were performed with HeLa cell cultures.