Here we report the expression, in the human ocular ciliary epithelium and in a human nonpigmented (NPE) ciliary epithelial cell line, of genes usually restricted to cone and rod photoreceptor cells of the retina. By RT-PCR and DNA sequencing we identified the expression of rhodopsin and components linked to its deactivation, including rhodopsin kinase, recoverin, and visual arrestin. We also detected the expression of transducin (T-alpha), phosphodiesterase (PDE-alpha), and cGMP-gated channel alpha -subunits. Cultured NPE cells responded to treatment with phorbol ester by enhancing the expression of rhodopsin mRNA three-to fourfold. Indirect immunofluorescence of the intact ciliary epithelium with monoclonal antibodies (MAbs) against rhodopsin, rhodopsin kinase, and visual arrestin revealed labeling preferentially restricted to the NPE cells. Furthermore, Western blot analysis of whole lysates from the pars plicata region of the human ciliary epithelium with MAbs demonstrated immunochemical cross-reactivity with proteins of molecular mass similar to rhodopsin (36 kDa), rhodopsin kinase (64 to 66 kDa), and arrestin (48-52 kDa) from the human retina. These results provide the first molecular evidence that components of a non-visual phototransduction pathway are expressed in the human ocular NPE ciliary epithelium, which may be linked to circadian entrainment tasks.