Inositol 1,4,5-trisphosphate (InsP3) mobilizes internal Ca2+ in cells by binding to a receptor protein, which has recently been purified and molecularly cloned. To clarify those neuronal functions that are regulated by InsP3, we have localized this InsP3 receptor protein immunocytochemically in the retina, a neural tissue of well-defined structure and function. Positive staining in neurons is confined almost exclusively to the synaptic layers. Using dissociated retinal neurons, we have further localized the receptor to presynaptic terminals of photoreceptors and bipolar cells, as well as the synaptic processes of amacrine cells. The specific association of InsP3 receptors with synaptic terminals suggests a role for InsP3 in synaptic modulation, especially with respect to transmitter release.