The staircases drawn by the isometric twitches up to the attainment of the steady-state at frequencies in the range of 1 to 60 min-1 have been studied in superfused half-ventricles of lizard, showing a normal inotropic state. The differences between the instantaneous twitch tension and the steady-state value has proved to be fitted by the algebraic sum of 3 or 4 exponential phases. The analysis of the electrical and mechanical activity of the preparation suggests a correlation between each exponential contribution to the staircase profile and the changes of the action potential duration as well as of the balance between the two sources (transmembrane influx and impulsive release from an intra or extracellular store) of calcium ions activating the contraction.