1. We analysed the effect of dantrolene (Dan) and five newly synthesized derivatives (GIFs) on Ca(2+) release from the sarcoplasmic reticulum (SR) of mouse skeletal muscle. 2. In intact muscles, GIF-0185 reduced the size of twitch contraction induced by electrical stimulation to the same extent as Dan. GIF-0082, an azido-functionalized Dan derivative, also inhibited twitch contraction, although the extent of inhibition was less than that of Dan and of GIF-0185. 3. In skinned fibres, Dan inhibited Ca(2+)-induced Ca(2+) release (CICR) under Mg(2+)-free conditions at room temperature. In contrast, GIF-0082 and GIF-0185 showed no inhibitory effect on CICR under the same conditions. 4. Dan-induced inhibition of CICR was not affected by the presence of GIF-0082, whereas it was diminished in the presence of GIF-0185. 5. GIF-0082 and GIF-0185 significantly inhibited clofibric acid (Clof)-induced Ca(2+) release, as did Dan. 6. Several Dan derivatives other than GIF-0082 and GIF-0185 showed an inhibitory effect on twitch tension but not on the CICR mechanism. All of these derivatives inhibited Clof-induced Ca(2+) release. 7. The magnitudes of inhibition of Clof-induced Ca(2+) release by all Dan derivatives were well correlated with those of twitch inhibition. This supports the notion that the mode of Clof-induced opening of the RyR-Ca(2+) release channel may be similar to that of physiological Ca(2+) release (PCR). 8. These results indicate that the difference in opening modes of the RyR-Ca(2+) release channel is recognized by certain Dan derivatives.