1. Single L-type calcium channels in chick ciliary ganglion neurons were studied at high current resolution in cell-attached patch recordings using quartz-glass micropipettes. 2. A single open-channel current amplitude was observed when Ba2+ was the charge carrier with a conductance of 26 pS at 110 mM barium. However, with 110 mM calcium two current fluctuation amplitudes were observed. These were termed low and high fluctuation amplitudes, CaL and CaH, and had conductances of 8.8 and 12 pS, respectively. These two levels probably reflect two different channel species. CaL was identified as an L-type calcium channel on the basis of resistance to inactivation, conductance, and dihydropyridine sensitivity. 3. Single-channel current fluctuations could be detected with calcium concentrations as low as 1.0 mM. Although the unitary conductance (gamma) was much greater with barium than calcium at every concentration tested, the concentration dependence of conductance was similar for gamma Ba, gamma CaH and gamma CaL. Fitting the concentration dependencies of these conductances with a Langmuir isotherm gave KD estimates of 4.7, 5.6 and 5.0 mM for barium, CaL and CaH, respectively 4. The single-channel conductance of the L-type channel (gamma L) can be described by the relation: conductance (in pS) = 9.2/(1 + 5.6/[Ca]) where [Ca] is the external calcium concentration in the 1.0-110 mM range. Thus, at a physiological external calcium concentration of 2 mM the conductance is 2.4 pS. 5. Ca2+ transport through the L-type calcium channel is particularly sensitive to changes in external calcium concentration in the physiological range but approaches saturation at about 10 mM. this characteristic may optimize the responsiveness of the cell to small changes in ambient calcium concentrations while limiting excess entry in the presence of abnormally high calcium levels.