The retina offers a model system for investigating the mechanisms that control cell type determination and differentiation in the vertebrate central nervous system. Previously, rod photoreceptor development in vitro was found to require a diffusible activity released by retinal cells (D. Altshuler and C. Cepko, Development 114, 947–957, 1992). In this report, we show that retinal-cell-conditioned medium and extracts contain two separable activities that influence rod development: a > 10 kDa inhibitory activity, and a stimulatory activity that is < 1 kDa and heat stable. Taurine was found to be a component of the < 1 kDa fraction and to stimulate rod development when added to retinal cultures. Taurine was not the only rod-promoting factor in these retinal preparations, however, as conditioned medium and extracts stimulated a higher level of rod development than did taurine alone. Taurine uptake into cells could be blocked without inhibiting taurine's ability to stimulate rod development, arguing against an osmoregulatory or nutritive mechanism of action. Finally, a competitive antagonist of taurine's bioactivity was identified and shown partially to inhibit rod development in retinal explants, suggesting that taurine may normally act to stimulate rod development in the retina. These results provide evidence for three activities, one of which is taurine, that are candidate regulators of rod photoreceptor development in vivo.