&NA;The effects of an acute increase in left ventricular systolic pressure and the effects of an intravenous isoproterenol infusion on myocardial (segment) lengthening rate and chamber (minor axis dimension) filling rate were examined in 12 anesthetized dogs. Measurements of left ventricular systolic pressure (by micromanometer) and of segment length and chamber dimension transients (by ultrasonic crystals) were made in variably afterloaded beats (threebeat descending aortic cross‐clamp) before and during an isoproterenol infusion that raised (+)dP/dt by 40%. During the baseline state, we found an inverse relation between the peak rate of increase in minor axis dimension [(+)dD/dt] and systolic pressure over a wide range of systolic pressures (110‐160 mm Hg) and end‐systolic dimensions (25‐40 mm); peak (+)dD/dt and end‐systolic dimension were also inversely related. During isoproterenol infusion, end‐systolic dimension fell from 29.7±3.1 to 28.0±3.1 mm and (+)dD/dt increased from 79.6±8.0 to 90.1±8.7 mm/sec; however, the slope and &lgr; intercept of the relation between (+)dD/dt and end‐systolic dimension were unchanged. Peak (+)dD/dt at a common end‐systolic dimension of 31 mm was nearly equal during baseline and isoproterenol states (64.2±6.3 vs. 65.1±6.6 mm/sec). Similar results were found using segment length transients. We interpret these data to indicate that (+)dD/dt is strongly influenced by changes in systolic pressure and dimension and that isoproterenol‐induced changes in (+)dD/dt are mediated, at least in part, through changes in systolic pressure and dimension. (Circulation Research1989;65:406‐416)