The proliferation kinetics and DNA content of thyroid follicular cells in rats were studied by autoradiography and cytophotometry. Continuous treatment of animals with methylthiouracil (MTU) results in hyperplasia followed by tumour growth in the thyroid gland. The mitotic index (MI) increases from 0.006 +/- 0.002% in controls to 0.13 +/- 0.06% in hyperplasia and to 0.09 +/- 0.03% in malignant cells. The same is true for the labelling index (LI) which rises from 0.08 +/- 0.003% in controls to 1.4 +/- 1.1% in hyperplasia and to 1.0 +/- 0.6% in follicular adenomas. The S-phase duration (TS) is shortened from 8.0 +/- 1.2 hr in controls to 6.0 +/- 1.4 hr in animals treated for 9 months with MTU and prolonged to 15.4 +/- 2.1 hr in papillary carcinomas. In all MTU-treated animals a decrease in the value of the potential population doubling time (TPD) and thyroid weight doubling time (TD) was observed. The cell loss factor (phi) decreases in animals treated for 3 months with MTU and increases during the stage of tumour growth in the gland (animals treated 12-15 months with MTU). DNA measurements in the nuclei of hyperplastic and neoplastic thyroid tissues reveal cells with values exceeding that in control animals. However, no difference was found in the DNA content between thyroid adenomas and carcinomas, nor between thyroid hyperplasia and neoplasia.