To examine whether a decrease in cytosolic Ca2+ affects fibrillation potentials, we studied effects of dantolene on these potentials in denervated rat muscle. Administered intraperitoneally, dantrolene sodium (12-22 mg/kg) abolished fibrillation potentials and subthreshold oscillating potentials over 2.5-6 h without affecting excitability of the muscle to electrical stimulation. Fibrillation potentials reappeared 36-96 h after administration of dantrolene. We suggest that cytosolic Ca2+ has a specific role in generating fibrillations.