Golgi-Cox impregnated loci of the thalamic reticular nucleus (TRN) of normal and neonatally T4-treated Wistar strain rats at 12, 20 and 30 days of age were analyzed. In a total of 120 TRN camera lucida drawings. The number of visible neurons, the area and the maximal transverse TRN length were quantitated. T4-treated rats showed a significant increase in the number of neurons at 12 days of age, followed by significant reductions of this parameter at 20 and 30 days old. By contrast the area of TRN hyperthyroid rats showed significant reductions on days 20 and 30 postnatally, and the maximal transverse length of the same group of rats showed a consistent significant reduction only at 30 days postpartum. The data are partly in line with previous studies showing an initial accelerated brain maturation, followed by a subsequent neuronal retardation, although the area and the maximal transverse TRN length measurements did not exhibit this sequence of development. The findings suggest that neonatal T4-treatment may interfere with the TRN morphological organization, and the modulatory actions upon the thalamic sensory transmission.