The effect of KCl concentration on the gating properties of the Ca2+ release channel in sarcoplasmic reticulum vesicles was studied through measurement of the permeation of choline and glucose by means of the light scattering method. The calcium concentration which activates the channel at micromolar range was shifted to higher concentration in the presence of KCl. This phenomenon can be explained by competition between Ca2+ and K+ at the Ca(2+)-binding sites. Five millimolar caffeine increased the apparent affinity of Ca2+ about 10 times but the effect of caffeine was not modified by K+. Inhibition of the channel by millimolar Ca2+ was also competed with by K+. These effects of K+ were not duplicated by choline, suggesting that the binding of choline to these sites is weak. Blocking of the channel by ruthenium red and Mg2+ was also competed with by K+, but blocking by procaine or tetracaine was not. After the opening of the channel by ryanodine, higher concentrations of these blockers were required to close the channel, and further, in the presence of K+ much higher concentrations of blockers were required. These results suggest that these drugs do not block the channel by plugging the pore directly but do so by regulating the gate, and that the Ca2+ release channel has many binding sites for activators and inhibitor to the gate.