In order to study the influence of hypothyroidism on the maturation of the visual cortex, rats were thyroidectomized (T) surgically at 10 days of age and injected with 131I within 2 days after the operation. These T rats, and age-paired controls (C), were killed at 20, 25, 30, 40 and 80 days of life. The groups of T rats were subdivided into 'severely' hypothyroid and hypothyroid animals, on the basis, respectively, of complete or incomplete stasis of body growth and depletion of pituitary growth hormone stores. The pyramidal cells of the visual coretx of 'severely' hypothyroid rats were studied separately from those of hypothyroid animals. It was found that the number of spines along the apical shaft of such cells was markedly and equally reduced in all of the T rats, as compared to C rats, irrespective of the degrees of hypothyroidism attained, and stopped increasing in number by 30 days of age. From this age onwards, the distribution of the spines could no longer be fitted by a mathematical model31 which, however, adequately described for normal rats the developmental changes which take place with increasing age in the distribution of spines along the shaft of pyramidal cells from the visual cortex. This lack of fit was only found for the 'severely' hypothyroid animals; data from the other hypothyroid animals could still be fitted by the model. Such data show that the above-mentioned model31 may be a useful tool for the study of the intensity of the effects of hypothyroidism on the maturation of cortical neurons.