The modifications of the extent and ultrastructure of the perivascular spaces during the development of blood vessel networks in the pars distalis of the adenohypophysis have been analyzed in normally developing chick embryos, from the 7th incubation day to hatching. The quantitative evaluation of the changes undergone by the perivascular spaces, carried out on semithin sections, showed that their extent gradually decreases and that this reduction is paralleled by an increment of the extent of the capillary bed. The ultrastructural study of the hypophyseal sinusoids allowed a precise determination of the size and the minute morphology of these spaces, which attain their definitive characters only during the late incubation stages, when the endothelial basement lamina forms.