Effects of Mg on transient depolarization (TD) and triggered activity (TA) induced by ouabain in guinea pig ventricular muscle were studied. Increased Mg concentrations (8-12 mM) suppressed the amplitude of TDs and prolonged the coupling intervals to the preceding action potential. Mg also increased the threshold of excitability for propagated action potentials and promptly terminated the otherwise sustained TA. The Mg-induced TA termination was due to both suppression of TDs and increases in the threshold of excitability, the latter appearing to take precedent.