Systolic time intervals (STI) were determined in 14 hypothyroid patients who did not have congestive heart failure or atrial fibrillation, and who were not taking cardiotropic drugs. Compared to normals, there was a significant decrease in the left ventricular ejection time (LVET) and there were significant increases in the pre-ejection period (PEP) and the PEP/LVET ratio. The prolongation of the PEP primarily reflected lengthening of the isovolumic contraction time. Serial recordings during thyroxine therapy in seven patients showed consistent changes in STI toward normal values. These results in hypothyroid patients complement our earlier data in hyperthyroid patients; they indicate that STI's are a sensitive measure of the heart as an end organ over the entire range of thyroid hormonal activity.