An anti-Fos protein antiserum was used to elucidate the diurnal expression of Fos protein in the normal and degenerate rd/rd mice retina. We have found that Fos expression is stimulated in cells of both inner nuclear layer (INL) and ganglion cell layer (GCL) at the onset of light period and reaches its maximum after 2 h. After which, the number of stained nuclei decreases along the light/dark cycle until almost no reaction is observed at the end of dark period. This expression pattern was similar in both normal and rd/rd mice although degenerate retinas showed a much lower number of stained nuclei. Aged rd animals also show Fos expression in GCL and INL in response to light stimuli suggesting that severely degenerate retinas are still able to transduce light stimulus.