The cell population in the thyroids of adult rats is highly differentiated and a closed one. The impairment of the proliferative capacity of the rat thyroid cells due to external Xand gamma-ray irradiation (Crooks et al, 1964; Malone et al, 1974) and uptake of high doses of radio-iodines (Greig et al, 1970) under goitrogenic stimulation have been investigated by various authors. In particular, Greig and his colleagues have studied the labelling indices with 3H-thymidine in goitrogenstimulated rat thyroids by pulse labelling technique (Greig et al, 1969). This article presents the labelling indices with 3Hthymidine at various depths in the thyroids of adult rats after in vivo irradiation with 18 MeV protons and under goitrogenic stimulation. The thyroids of adult male rats (strain Wister) weighing 230 ± 20 g were irradiated with a single integrated dose of 11.7 Gy by the 18 MeV proton microbeam of the AEGP Kompaktzyklotron at the Technische Universität Berlin. Proton dose in the gland was calculated from the energy spectrum of the protons, assuming the thyroid lobes to be a pair of hemispheres of 3 mm diameter of soft-tissue-equivalent material (Mukherjee, 1982).