Bovine retinal cyclic nucleotide-gated (CNG) ion channel contains an evolutionary conserved N-glycosylation site in the external loop between the fifth transmembrane segment and the pore-forming region. The effect of tunicamycin treatment and the site-specific mutation suggested that the channel is glycosylated when expressed in Xenopus oocytes. To test the role of glycosylation in this channel, N-glycosylation was abolished by mutation, and the detailed permeation and the gating characteristics of the mutant channel were investigated. The charge contribution turned out to be detectable, although the mutation of the N-glycosylation site did not affect expression and functionality of the CNG channel in oocytes. (C) 2000 Federation of European Biochemical Societies.