Administration of triiodothyroacetic acid (triac) to pregnant rats produces cardiac hypertrophy and myofibrillar disarray in the hearts of the newborn offspring. Previous experiments have shown that concurrent administration of dl exprenolol or dl or d propranolol prevent the disarray but not the hypertrophy, suggesting that membrane stabilization and not beta-adrenergic blockade may be responsible for modifying the actions of triac. In order to clarify this, further experiments have been carried out whereby timolol, a beta-blocking agent with minimal or no membrane stabilizing activity, and procainamide, a pure membrane stabilizing compound, have been administered with triac. Timolol had no effect, but procainamide reduced the level of disarray and the hypertrophy to a minor degree. The results have thus confirmed the proposal that membrane stabilization is of major importance in modifying the actions of triac.