The present study uses cell-attached patch-recording techniques to study the single-channel properties of Ca2+ channels in isolated salamander photoreceptors and investigate their sensitivity to reductions in intracellular Cl-. The results show that photoreceptor Ca2+ channels possess properties similar to L-type Ca2+ channels in other preparations, including (1) enhancement of openings by the dihydropyridine agonist, (-)BayK8644; (2) suppression by a dihydropyridine antagonist, nisoldipine; (3) single-channel conductance of 22 pS with 82 mM Ba2+ as the charge carrier; (4) mean open probability of 0.1; (5) open-time distribution fit with a single exponential (tau0 = 1.1 ms) consistent with a single open state; and (6) closed time distribution fit with two exponentials (tau(c1) = 0.7 ms, tau(c2) = 25.4 ms) consistent with at least two closed states. Using a Cl- -sensitive dye to measure intracellular [Cl-], it was found that perfusion with gluconate-containing, low Cl- medium depleted intracellular [Cl-]. It was therefore possible to reduce intracellular [Cl-] by perfusion with a low Cl- solution while maintaining the extracellular channel surface in high Cl- pipette solution. Under these conditions, the single-channel conductance was unchanged, but the mean open probability fell to 0.03. This reduction can account for the 66% reduction in whole-cell Ca2+ currents produced by perfusion with low Cl- solutions. Examination of the open and closed time distributions suggests that the reduction in open probability arises from increases in closed-state dwell times. Changes in intracellular [Cl-] may thus modulate photoreceptor Ca2+ channels.