Systolic time intervals and drug plasma concentrations have been measured in a group of patients receiving repeated treatment with nortriptyline. Significant positive correlations between plasma nortriptyline levels and prolongation of pre-ejection phase (P less than 0.005)) and increase in the ratio pre-ejection phase, left ventricular ejection time (P less than 0.05) were obtained. A deterioration in cardiac function, with increase in heart rate, resulting in a negative inotropic effect has been shown to occur with therapeutic doses of nortriptyline. The potential dangers of tricyclic antidepressant drugs on the heart in patients whose myocardium is already compromised or those who accumulate high plasma concentrations are emphasised.