It has been shown that inhibition of phosphodiesterase (PDE) or an increase in intracellular cyclic adenosine monophosphate (cAMP) can lower the intraocular pressure in mammalian eyes. The PDE inhibitor rolipram binds with high affinity to a cAMP-dependent PDE. In the following study, receptors for rolipram in the mammalian eye are determined by autoradiography. About 1050 histological sections of rabbit (albino and pigmented), rat, monkey (baboon and marmoset) and human eyes were examined concerning rolipram-binding sites in different structures. The highest specific binding of rolipram to the ciliary body epithelium was seen in human, rabbit and baboon eyes. Marmoset eyes showed the highest specific binding in the ciliary body muscle, rat eyes in the retina. Specific binding was consistently high in the ciliary body epithelium and therefore at the site of aqueous humor formation. Because of these results and the known mode of action of rolipram, one can assume a close connection between rolipram-binding sites and the regulation of intraocular pressure.