AbstractThe amplitude of caffeine contractures, tissue redox‐state potential, and electrical activity were simultaneously measured in the sartorius muscle of the frog,Rana esculentafollowing oxidant (methylene blue) and reductant (ascorbate, hydroquinone) treatments.Pretreatment with oxidant, in parallel with the increment of redox‐state potential, increased, while pretreatment with reductant, in parallel with the decrement of redox‐state potential, decreased significantly the amplitudes of caffeine contractures. The electrical activity, registered simultaneously, was not changed significantly.The mechanism of action of caffeine contractures is related to the increase of cytoplasmic Ca2+, and the results indicate that the actual redox‐state potential level, as one of the determinants of the internal milieu, is also a regulatory factor in relation to the excitation‐contraction