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Contractile activation in response to application of different perfusing solutions was observed in developing myotomes from larvae of Xenopus laevis. Contractures resulted from treatment with isotonic solutions containing high K+ concentrations in myotomes from embryos from stage 24 onwards. These developed over 5-10 s and inactivated over 20-30 s. Contractures persisted in curarized preparations. The applied K+ concentrations at the estimated mechanical threshold in embryos at stages 26 and 34 were about 30 and 20 mM respectively. Maximum activity was achieved at K+ concentrations of 78 and 48 mM respectively. The lyotropic ion thiocyanate (10 mM) potentiated K+ contractures, and shifted the threshold K+ concentration to lower values. Contractures persisted in the presence of 3 mM-Mn2+ and in Ca2+-free solutions, with unaltered mechanical thresholds. K+ contractures were reversibly abolished by 2 mM-tetracaine. Comparisons of resting potentials in 3 and 20 mM-K+ confirmed that the resting potential remained sensitive to external K+ concentration, whether in control, tetracaine-containing, or low-Ca2+ bathing solutions. Caffeine (10-20 mM) caused sustained contractures from stage 24 onwards. Caffeine sensitivity was greatest at stages 26-28, then appeared to decline at stages 33-36. These observations suggest that mechanisms for excitation-contraction coupling develop in Xenopus embryos at the same time as do completed sarcomeres, close to stage 24. Activation of contraction then assumes the adult pattern, involving voltage-dependent release of the intracellular store of activator, independent of entry of extracellular Ca2+.