Short exposure of frog atrial trabeculae to high [K] fluids induces a large transient increase ('facilitation') of subsequent twitch tension. This facilitation declined during regular stimulation with a time course similar to that of the initial 'staircase' phenomenon following a change in heart rate. The facilitation increased in a graded manner, above a threshold level, with the potassium concentration in the conditioning high [K] fluid. Variation of external calcium during the high [K] treatment did not significantly affect the height of the facilitated twitches. However, variation of external calcium after the conditioning treatment produced large changes in tension. More detailed study of this effect showed that the facilitatory effect could be entirely explained by an increased sensitivity of heart cells to the instantaneous level of external calcium concentration. The results suggest that twitch facilitation is brought about by the enhancement of calcium inward movement associated with an action potential.