1. The effects of various concentrations of lanthanum (La3+) on the force of contraction and on action potentials was investigated in isolated papillary muscles of the guinea-pig. 2. The force of contraction was initially reduced by La3+. During long exposure to high concentrations (i.e. 50, 100 and 500 muM) the contraction amplitude was increased again and a rise in resting tension was observed. These late changes in force of contraction also occurred after short exposure to 500 muM La3+ (2 min) when La3+ had been removed from the medium. 3. The action potentials were shortened at 20% of repolarization (plateau level) during La3+ exposure, whereas at 90% of repolarization a transient prolongation was observed. The resting potentials were reduced only at high concentrations and long duration of exposure. 4. 500 muM La3+ reduced the force of contraction to the control value obtained before addition of ouabain (5x10(-7)M). The ouabain-induced shortening in action potential duration at the plateau level was not reversed by the same concentration of La3+. 5. It is concluded that the action of La3+ on cardiac muscle cannot be explained by a simple displacement of superficially bound calcium. The characteristic effects observed at high concentrations reduce the value of La3+ as a pharmacological tool used in the study of calcium turnover.