Levels of dopamine and melatonin exhibit diurnal rhythms in the rat retina. Dopamine is high during daytime adapting the retina to light, whereas melatonin is high during nighttime participating in the adaptation of the retina to low light intensities. Dopamine inhibits the synthesis of melatonin in the photoreceptors via Drd4 receptors located on the cell membrane of these cells. In this study, we show by semiquantitative in situ hybridization a prominent day/night variation in Drd4 expression in the retina of the Sprague-Dawley rat with a peak during the nighttime. Drd4 expression is seen in all retinal layers but the nocturnal increase is confined to the photoreceptors. Retinal Drd4 expression is not affected by removal of the sympathetic input to the eye, but triiodothyronine treatment induces Drd4 expression in the photoreceptors. In a developmental series, we show that the expression of Drd4 is restricted to postnatal stages with a peak at postnatal day 12. The high Drd4 expression in the rat retinal photoreceptors during the night supports physiological and pharmacologic evidence that the Drd4 receptor is involved in the dopaminergic inhibition of melatonin synthesis upon light stimulation. The sharp increase of Drd4 expression at a specific postnatal time suggests that dopamine is involved in retinal development.