The purpose of this controlled study was to study the effect of cimetidine and ranitidine on the myocardium of 8 healthy male volunteers using systolic time intervals. Ranitidine caused a significant decrease in the QS2I, as from 40 min indicating a positive inotropic effect. There were no significant changes in heart-rate or blood-pressure. It is postulated that ranitidine, differing structurally from cimetidine, may have a higher affinity for H2-presynaptic receptors at the sympathetic myocardial junction resulting in an increase in noradrenaline and an increased inotropic effect.