Barium ions are able to substitute for calcium as charge carriers for the slow inward current of cardiac membrane. The effects of such a substitution on the mechanical activity of voltage-clamped frog atrial trabeculae have been investigated using the double mannitol gap device coupled with a mechano-electric transducer. In the presence of barium, the 'tonic' phase of the contraction is suppressed and the remaining contraction is correlated to the influx of barium ions via the slow inward channel. Since a Na-Ca exchange regulates the tonic contraction, the present results suggest that barium ions cannot substitute for calcium in this mechanism.