Thalamofugal visual projections of light-incubated chicks are organised asymmetrically. This asymmetry, which is generated by light stimulation of the embryo during the final days of incubation, is sexually dimorphic, being more pronounced in males than in females. We have shown that the development of the asymmetry can be prevented by elevating circulating levels of 17 beta-oestradiol in the embryo prior to hatching. Light-incubated male chicks were treated 5 days prior to hatching with either one of two doses of 17 beta-oestradiol (1.5 mg or 2.5 mg in a 0.1 ml solution of 10% ethanol in olive oil) or the vehicle only. After hatching the retrogradely labelling fluorescent dyes True Blue and Fluorogold were injected into the left and right side of the hyperstriatal region of the forebrain, consequently labelling the cell bodies of thalamic neurons which project to this region. Although a pronounced asymmetry was present in the control group, it was not present in both of the 17 beta-oestradiol-treated groups. These results suggest that the asymmetrical development of thalamofugal visual projections in response to lateralised light stimulation of the embryo is dependent on circulating levels of steroid hormones, and that compared to the male, the lesser degree of asymmetry found in thalamofugal projections of untreated females may be related to the higher levels of circulating oestradiol present in females prior to hatching.