Caffeine and theophylline evoke maintained tension in tonic skeletal muscle fibers of the frog. Their effects depended upon concentrations ranging from 0.5 to 14 mM. The resting potential of tonic skeletal fiber was unaffected by caffeine or theophylline (4 mM). Caffeine and theophylline contractures have a peak tension followed by a sustained tension, which recovered spontaneously when returned to normal solution. The peak tension and total tension (the area under tension-time curve) were reduced when the fibers were soaked in calcium-free solution. In addition the tension was reduced by calcium channel blockers (cadmium). The sustained tension was increased when external calcium was raised. These results suggest that caffeine and theophylline promote the calcium release from sarcoplasmic reticulum and probably the entry of calcium from external medium.