1. Pharmacological blockers of calcium-induced calcium release from isolated skeletal sarcoplasmic reticulum (SR) vesicles have been introduced into frog skeletal muscle fibres to determine their effects on excitation-contraction coupling. 2. Among the blockers tested, Ruthenium Red, neomycin, gentamicin and 9-aminoacridine inhibited the SR Ca2+ release associated with excitation-contraction (E-C) coupling as much as they inhibited caffeine potentiation of that release. Protamine, certain of its derivatives, and spermine were ineffective in both in situ tests. 3. Alternative sites of polyamine action on the contractile proteins, SR Ca2+ uptake or charge movements were ruled out. 4. All polyamines tested required considerably higher concentrations to inhibit excitation-contraction coupling than to block Ca2+ release from isolated SR vesicles. 5. The quantitative pharmacological difference in sensitivity between isolated and intact systems serves as a reminder that results on isolated systems cannot generally be used to predict results of the same substances on more physiological systems. 6. Since caffeine is known to open the SR 'Ca2+ release channels' (the ryanodine receptors that mediate Ca(2+)-induced Ca2+ release), the equal effectiveness of these blockers at inhibiting excitation-contraction (E-C) coupling and its potentiation by caffeine suggests that the SR 'Ca2+ release channels' are indeed involved in excitation-concentration coupling in skeletal muscle, although the results do not indicate how the channel is gated open during E-C coupling.