The pharmacological characteristics and the anatomical localization of [3H]-SCH 23390 in sections of the ciliary body of the rabbit eye were analyzed using a radioreceptor assay and autoradiographic techniques. [3H]-SCH 23390 was bound to sections of rabbit ciliary body in a manner consistent with the labelling of D1-like receptor sites. The dissociation constant (Kd) was 0.62 nmol/l, while the maximum binding capacity (Bmax) was 117 +/- 9 fmol/mg tissue. Light microscope autoradiography revealed [3H]-SCH 23390 binding sites within the epithelium of the ciliary processes, which is the ocular structure involved in the secretion of aqueous humor. No specific accumulation of silver grains was noticeable within the iridocorneal angle, which is the structure involved in the outflow of aqueous humor. These findings suggest that the rise in intraocular pressure caused by D1 receptor agonists is probably mediated by an increase of aqueous humor formation rather than by an inhibition of the outflow of aqueous humor.