Previous RT-PCR experiments revealed the expression of gene transcripts for a variety of aquaporins in the neural retina, including aquaporin-0. We investigated by immunohistochemistry and Western blotting whether the aquaporin-0 protein is expressed in the retina of the rat. In addition to the lens, immunoreactivity for aquaporin-0 was expressed in the neural retina, but was absent in the pigment epithelium, choroidea, and sclera. In the neural retina, aquaporin-0 immunoreactivity was expressed by the nuclei and the synaptic terminals of protein kinase alpha- and beta-expressing bipolar and amacrine cells, and by the nuclei of neuronal cells in the ganglion cell layer. The immunoreactivity for aquaporin-0 did not co-localize with calbindin, a marker of horizontal cells, or with aquaporin-4, the glial water channel. Transient retinal ischemia caused a slight decrease in the retinal content of aquaporin-0, likely by degeneration of protein kinase alpha-expressing bipolar cells. It is concluded that aquaporin-0 may be involved in the regulation of the activity of retinal second order neurons.