Dependency of astrocytes on thyroid hormones during development was studied in intraocular cortex cerebri grafts, using antibodies to the glial fibrillary acidic protein (GFA). Fetal cortex cerebri was left in the anterior eye chamber of normal and thyroidectomized adult recipient rats 3 and 8 weeks to mature. All cortex grafts survived and became vascularized in the eye. The grafts were cryostat-sectioned, and reacted with an antiserum specific for GFA. The density of GFA immunoreactivity in the cortex graft neuropil was subjectively estimated with fluorescence microscopy. After 3 weeks in oculo, the number of positive structures was significantly lower in the thyroidectomized group as compared to the euthyroid control group. This effect of thyroidectomy on the cortex grafts was entirely counteracted by daily injections of thyroxine (100 μg/kg s.c.) to similarly thyroidectomized recipients. The distribution of GFA-positive structures was even within individual grafts of all three groups. Neither were any differences in morphology of fluorescent astroglial processes observed. The overall density of fluorescent GFA-positive structures was considerably higher than that found in age-matched normal in situ counterparts, even in the thyroidectomized group. After 8 weeks in oculo, the difference between the control and thyroidectomized groups in density of GFA-positive structures in the neuropil had disappeared, and no general increase in density of immunoreactivity was noted in the control group. These results indicate that thyroid hormones might normally play a role for the development of GFA in the early postnatal brain, as thyroid hormone deficiency causes a transient delay in development of GFA immunoreactivity in intraocular cortex cerebri grafts.