We have previously shown (Ruiz-Marcos et al. 1980, 1982) that thyroidectomy (T) performed in rats at 40 days of age, well past the neonatal period of development, results by 80-90 days of age in a decrease of the number of spines along the shaft of pyramidal neurons with the cell body in layer V in the visual area of the cerebral cortex. We have here studied how soon after the operation an effect on spine number and distribution may be observed. We have found that the response of these neurons to T is very rapid: a decrease in the number of spines/shaft between T and age-paired controls (C) rats is statistically significant by the earliest period of observation, namely 5 days after T. These results may be related to those of Dembri et al. (1983) showing that T performed in adult rats decreases the activity of Type I RNA polymerase by 5 days after the operation. It is possible that T impairs the synthesis of some compound(s) necessary for the formation and maintenance of spines. The present results suggest that spine number is not a fixed structure of the apical shaft once brain development is over, but is in a state of continuous formation and degradation. We have further observed that the effect of T performed at 40 days of age is more pronounced in the distal part of the shaft than on the rest, a result similar to that found after neonatal T (Ruiz-Marcos et al. 1982). However, contrary to findings after early hypothyroidism, T at 40 days of age does not distort the distribution of spines along the shaft.