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Toxin γ from the venom of Tityus serrulatus scorpion produces a partial block of the surface Na channel in frog muscle. This block occurs with no change in the voltage-dependence or in the kinetics of the remaining surface Na current. The partial blockade of Na channel activity occurs with no change in tubular Na currents nor in twitch tension. The maximum effect of the toxin is attained at concentrations as low as 3×10 M. Hyperpolarization to potentials more negative than the resting potential ( E=−90 mV) reduces or abolishes the effect of the toxin. Radioiodinated toxin γ binds to frog muscle membranes with a very high affinity corresponding to a dissociation constant of about 1×10 M. Data obtained with both rabbit and frog muscle indicate that toxin γ is specific for Na channels in surface membranes. Toxin γ does not seem to bind to Na channels in T-tubule membranes. The biochemical data are in good agreement with electrophysiological studies and data on contraction. There is one Tityus γ toxin binding site per tetrodotoxin binding site in surface membranes. Competition experiments have confirmed that Tityus γ toxin binds to a new toxin receptor site on the Na channel structure. This site is the same that the toxin II from Centruroides suffusus binding site, but this toxin has 100 times less affinity for the Na channel than Tityus γ toxin. [ABSTRACT FROM AUTHOR]