Cyclic nucleotide-gated channels are important in visual and olfactory transduction, and possibly in other neuronal functions. These channels have a complex permeability to Ca2+ ions that may be important in their cellular functions. They are composed of two different subunits, alpha and beta, that have been cloned and expressed, but the beta subunit alone cannot be activated by cyclic nucleotides, confounding the analysis of its characteristics. However, we found that nitric oxide can activate the homomeric expressed beta subunit, and the resulting channel possesses many properties of the L-type Ca2+ channels, including high permeability to Ca2+ ions and sensitivity to Ca2+ channel blockers. Thus, the Ca2+ permeability characteristics of native channels are mostly conferred by properties of the beta subunit, and the beta subunit alone can act as a NO-sensitive Ca2+ channel. A nearly identical conductance activated by NO is present in the membrane of rat vomeronasal neurons, indicating that homomeric beta channels exist in vivo.