The force-interval relationship the dependence of cardiac contractility on the rate and pattern of stimulation has been shown to be independent of preload, but sensitive to the inotropic and disease state of the heart. The force-interval relationship was evaluated for the left ventricles of 42 patients, ages 1.5- 20 years, during cardiac catheterization using a micromanometer pressure transducer-tipped catheter and an atrial pacing electrode catheter. The left ventricular (LV) minor axis was monitored echocardiographically, and the end-diastolic dimension (EDD) and posterior wall thickness (PW) were measured. Hearts were paced at a variety of basic cycle intervals, to (1/heart rate), and two test stimuli were introduced at various times during a pause in the regular stimulation (test intervals t, and t2 were measured relative to the last regular systole before the pause). For t, < to and t2 2 tl + to Pma of the second test systole was greater than Pma for the last regular systole before the pause (equivalent to postextrasystolic potentiation). The ratio of these values of Pmax (test to regular) for systoles with equal LVEDDs, the force-interval ratio, was used to characterize the relationship. Patients were divided into three groups: group 1 (n = 17) patients had normal LVEDD, LVPW and pressure indices. Group 2 (n = 17) patients had increased LVEDD or LVPW but normal pressure indices. Group 3 (n = 4) patients had increased LVEDD and depressed pressure indices. The force-interval relationship was similar for groups I and 2: Pmax increased monotonically with t, or t2. However, the force-interval ratio was larger for group 2. The relationship for group 3 was different. Pmaz was a biphasic function of the test interval, increasing for small values of t, or t2 and decreasing for larger values of t,. The force-interval ratio was smaller than or equal to unity for short values of t,. This postextrasystolic depression was never found in groups I or 2.