It has been shown that quinine, a bitter tasting substance, induces a cationic membrane current in frog taste receptor cells. We analyzed the quinine-induced current fluctuation to further characterize the cationic channel conducting the current. Taste cells were enzymatically isolated from fungiform papillae of the bullfrog tongue. Membrane currents were recorded under voltage-clamp by using the whole-cell patch clamp technique. A Cs(+)-containing pipette solution was used to block K+ currents. When quinine was bath applied, it induced a steady inward current in cells held at -60 mV. The current data were digitized at a sampling frequency of 2 kHz after having been low-pass filtered (cut-off frequency, 1 kHz). The spectral density distribution was calculated by 1024 points fast Fourier transform (FFT), and was fitted by a sum of two Lorentzian functions, with corner frequencies at 10-20 Hz and at 90-120 Hz. The single-channel conductance calculated from the variance versus mean current plot was 5 pS in a normal saline solution containing 1.8 mM Ca2+. In a nominally Ca(2+)-free solution, the single-channel current was doubled. Our previous study has shown that the quinine-induced current is enhanced in low extracellular [Ca2+]. This enhancement could be partly attributable to the Ca2+ suppression of the single-channel conductance.