Fast-twitch and slow-twitch rat skeletal muscles produce dissimilar contractures with caffeine. We used digital imaging microscopy to monitor Ca²⁺ (with fluo 3-acetoxymethyl ester) and sarcomere motion in intact, unrestrained rat muscle fibers to study this difference. Changes in Ca²⁺ in individual fibers were markedly different from average responses of a population. All fibers showed discrete, nonpropagated, local Ca²⁺ transients occurring randomly in spots about one sarcomere apart. Caffeine increased local Ca²⁺ transients and sarcomere motion initially at 4 mM in soleus and 8 mM in extensor digitorum longus (EDL; ~23°C). Ca²⁺ release subsequently adapted or inactivated; this was surmounted by higher doses. Motion also adapted but was not surmounted. Prolonged exposure to caffeine evidently suppressed myofilament interaction in both types of fiber. In EDL fibers, 16 mM caffeine moderately increased local Ca²⁺ transients. In soleus fibers, 16 mM caffeine greatly increased Ca²⁺ release and produced propagated waves of Ca²⁺ (~1.5-2.5 µm/s). Ca²⁺ waves in slow-twitch fibers reflect the caffeine-sensitive mechanism of Ca²⁺-induced Ca²⁺ release. Fast-twitch fibers possibly lack this mechanism, which could account for their lower sensitivity to caffeine.