Summary Thyroid follicular cell proliferation in normal and MTU-treated male newborn, 10, 60 and 340-day-old Wistar rats was studied. In methylthiouracil(MTU)-treated newborn animals the thyroid weight growth curve consists of 2 phases: an almost exponential phase for approximately 8 days and a plateau phase between the 10th and 24th day. In 10, 60 and 340-day-old animals the growth pattern of the thyroid weight is presented by a lag phase of 2 days, an exponential phase for about 13 days and a plateau phase until the end of the experimental period (24 days). By means of autoradiography it was found that during the first days after birth between 7 and 8 per cent of all follicular cells are labeled. The labeling index curve showed a sharp decrease after the second week and then reached an unchanged level in 60-day-old rats. About 0.2 per cent of all follicular cells in 60 and 340-day-old animals were labeled. After MTU-treatment in all experimental groups the curves indicated a sharp increase of labeled follicular cells between the 2nd and 3rd day with a peak at the 8th day. Between the 8th and 24th day a considerable decrease of labeled cells was observed. The highest value for labeled follicular cells was 19 per cent in the newborn group, 14.5 per cent in 10-day-old animals, 8.4 per cent in 60-day-old animals and 4.3 per cent in the 340-day-old group. The rate of follicular cell labeling after MTU-treatment is 3,2, 40 and 20 times larger in newborn, 10, 60 and 340-day-old animals, respectively. From the labeled mitosis curve some cell cycle parameters of the follicular cells in 10 and 340-day-old animals were established. Thus, in the 10-day-old group the duration of the DNA-synthesis period (Ts) was 7.5 hrs, of the DNS-postsynthetic (premitotic) period (G 2) 3 hrs and of the mitosis (T M)-period 4.5 hrs. The same phases of the cell cycle in the 340-day-old group were as follows: Ts-10 hrs, T G 2–3 hrs, T M-7.5 hrs. Factors affecting the thyroid cell proliferation during postnatal development are discussed.