The development of the blood brain barrier (BBB) and the vessel permeability to horseradish peroxidase (HRP) have been analyzed in the optic tectum of chick embryos developed under normal and hypoxic conditions, normal chickens, and chickens born from fertilized eggs incubated under hypoxia but kept in the open air after hatching. The development of chick embryos under a situation of chronic hypoxia was obtained by covering, a half of the shell of fertilized eggs with a thick layer of melted paraffin to obtain a reduction of the exchanges normally occurring between embryonic blood vessels and open air. In the tecta developed in normal conditions the BBB to HRP begins to form on the 14th i.d. and it is complete on the 17th i.d. The O2 deprivation, producing remarkable alterations of the neural substratum, does not affect the development of the BBB to HRP, since in chicks of 17 i.d., grown up under hypoxic conditions, the tectal microvessels are not permeable to the tracer, being it mainly confined within the vessel lumina. Nevertheless in specimens kept under hypoxia until hatching, areas of perivascular spread of the marker have been observed corresponding to the vessel wall tracts presumably damaged by the experimental conditions along which the BBB to HRP is not complete.