Cyclic-nucleotide gated channels (CNG) play an important role in the signal transduction of photoreceptors and olfactory cells. Native olfactory channels are heterotetramers composed of three homologue subunits: 2 X CNGA2, 1 X CNGA4, 1 X CNGB1b. The channels are activated by the direct binding of cyclic nucleotides to intracellular binding domains. Because the macroscopic current, reflecting the collective activity of many channels can lead to erroneous conclusions (Ruiz et al, 1999), it is straightforward to examine the gating of single homo- und heterotetrameric channels in order to learn more about the complex mechanism of channel activation, i.e. how the ligand binding is transformed into opening of the pore. We expressed homo- and heterotetrameric CNG channels in Xenopus oocytes and examined in inside-out patches, containing one and only one channel, the gating kinetics under steady-state conditions, at saturating and four subsaturating cGMP concentrations. In an extensive analysis we considered the single-channel current amplitude, the open-channel noise, the open probability as well as, closed and open times. Homotetrameric channels produced a single amplitude of the unitary current which is 5.0±1.0 pA at +100 mV. The open time histograms at the different cGMP concentrations were compared with the predictions of the C4L-Model, derived from ligand binding and activation gating in macroscopic currents. In the heterotetrameric channels we observed a dominant small open level of 1.4±0.2 pA at +100 mV. These small levels very consistently superimposed by very short larger levels which were only rarely long enough to determine the amplitude. These larger levels suggest that the heterotetrameric channel reaches only very briefly the conductance state of the homotetrameric CNGA2 channel. The presented data contribute to an improved understanding of the role of the CNGA4 and CNGb1b subunits in the heteromultimeric channels.