Short-term, repeated myocardial ischemia was produced in 20 opened-chest anesthetized dogs by left anterior descending (LAD) branch occlusion. Left ventricular pressure (LVP) and its dP/dt were recorded by Satham SF-1 transducer, and isometric local force (F) was measured by a Walton-Brodie strain gauge arch sewn onto LV free wall. In all experiments, within a few beats after LAD occlusion, peak negative dP/dt started to diminish its amplitude, reaching a nadir after 20-30 sec (63 +/- 5% of control, 9 dogs). It recovered to 83 +/- 3% of control level in 45 sec and remained essentially unchanged thereafter. This change preceded those of LVP and max dP/dt, which showed only minor falls. In 6 dogs, changes by occluding root of LAD were compared with those by its branch occlusion. The greater the extent of ischemia, the more marked and prolonged were the changes. While F in the ischemic center (Fc) rapidly lost its contractile activity following LAD occlusion, F in marginal zone between infarcted and non-infarcted region (Fm) showed variable extent and different time-course of its diminution. Thus, it was concluded that the change in peak negative dP/dt following LAD occlusion was one of the sensitive signs of early ischemic changes, and was attributed at least in part to characteristic changes in contractile activity of myocardium surrounding the ischemic injury.