The role of calcium in the pathogenesis of hypertrophic cardiomyopathy was investigated experimentally. For this purpose the calcium antagonist verapamil and the calcium ionophore A23187 were administered to pregnant rats together with triac in order to observe their effect on triac induced myocardial disarray and hypertrophy in developing rat hearts. At a low dose verapamil reduced both the level of disarray and hypertrophy, but a higher dose produced hypertrophy when given alone. A23187 did not appreciably potentiate the actions of triac when given in combination but when administered alone produced both disarray and hypertrophy. Verapamil prevents the inward movement of calcium ions to the myocardial cell, whereas A23187 increases the inflow of calcium ions. The results suggest that the actions of triac in producing myocardial disarray and hypertrophy are attributable to an increased concentration of intracellular calcium.