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METHYLTHIOURACIL administration causes the rat thyroid gland to undergo an increase in weight (the goitrogenic response) (Thyssen, 1947;Santler, 1957; Crooks, Greig, Macgregor and McIntosh, 1964). Doniach and Logothetopoulos (1955) observed that the goitrogenic response could be inhibited by ionising radiation delivered by radioiodine. Crooks et al. (1964) used the degree of inhibition of the goitrogenic response as an indicator of the effect of graded doses of X-rays on the thyroid cell population by comparing the growth pattern of irradiated rat thyroids with that of unirradiated controls. They found that the normal growth curve was biphasic. Furthermore, on the basis of observed differing radiosensitivities of the two phases they suggested that the shape of the growth curve might be accounted for by a predominance of cell hypertrophy in the first growth phase and a predominance of cell hyperplasia during the second. However, Thyssen (1947) reported a different form of growth curve of the rat thyroid following methylthiouracil administration consisting of a lag phase, a phase of logarithmic growth followed by a plateau phase.In view of these conflicting reports it was decided to investigate the normal goitrogenic response of the rat thyroid gland to methylthiouracil administration and by recourse to quantitative histological techniques to see what relation existed between the increase in thyroid weight and the increase in the follicular cell population and follicular cell volume. It was hoped thereby to find a better model for investigating the effects of ionising irradiation on mammalian cells in vivo. MATERIALS AND METHODSMethylthiouracil powder was dissolved in weak sodium hydroxide (100 g. in 200 ml. N/II NaOH) and then made up with 1 % sucrose (w/v) to a final concentration of 0-1 %.Male Wistar rats weighing 220 to 280 g. were allowed to drink at will the 0*1 % solution of methylthiouracil in 1 % sucrose. The sucrose encouraged the animals to drink (Crooks et al., 1964). Control animals received the 1% sucrose solution only. Fresh sucrose and methylthiouracil solutions were prepared every second day. Sample groups of test and control animals were killed before commencing methylthiouracil administration and at intervals thereafter. The precise arrangements were as follows:Experiment P: A pilot study was carried out with 14 randomly constituted groups of 5 rats each. There were 20 animals to each cage and the animals were marked distinctively according to group. Two groups (10 animals) were killed

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