Nasal and temporal microexplants from the retina of the chicken embryo (E6; White Leghorn) were cultured on a collagen gel in the presence of extract from anterior or posterior E18 optic tectum. After 4 days of incubation maximum neurite lengths were measured as a function of protein concentration (1-500 micrograms/ml). Temporal retina was stimulated by both extracts but maximum neurite growth was obtained with anterior optic tectum at 500 micrograms/ml. Conversely, nasal retinal fiber outgrowth was best stimulated by posterior tectum, with maximum neuritic extension occurring at a concentration of 60 micrograms/ml. The stimulation was exerted by soluble protein fractions. The results show that topographically distinct axonal target fields of the optic tectum can selectively stimulate growth of neurites from the appropriate region of the retina.