Adenosine is present in all cells and body fluids and has been suggested to play several roles in the physiology of ocular tissues. The present study was undertaken to determine which types of adenosine receptor mRNAs are present in the rat eye, and where they are expressed. RNA or deoxyoligodeoxynucleotides complementary to rat adenosine receptor subtypes A1, A2A, A2B and A3 were used to generate 35S labeled antisense and sense probes. The probes were then used for in situ hybridization on 10 microm cryosections of the rat eye including the cornea, iris, ciliary body, lens, retina, choroid and sclera. A1, A2A and A2B receptor mRNAs were demonstrated in the ciliary processes. A1 receptor mRNA was also expressed in the ganglion cell layer of the retina. The retina also showed A2A receptor mRNA expression, which was most prominent in the inner nuclear layer and less prominent in the ganglion cell layer and outer nuclear layer. Weak A2A expression was found in the retinal pigment epithelium and choriocapillaris. No significant expression of A3 receptor mRNA was found in the rat eye. In conclusion, using in situ hybridization, we have demonstrated expression of mRNA for A1, A2A and A2B adenosine receptors in the rat eye. The expression patterns support specific roles for adenosine in the ciliary process and retina. Copyright 1997 Academic Press Limited.