We assessed the effect of felodipine on left ventricular (LV) performance in 10 healthy, conscious, chronically instrumented dogs, using load-independent measures derived from variably loaded pressure-volume relations. With all reflexes intact, felodipine (25 nmol/kg, intravenously) caused significant decreases in LV end-systolic pressure (132 +/- 13 vs. 109 +/- 14 mm Hg, P less than .05) and effective arterial elastance (11.1 +/- 1.6 vs. 8.7 +/- 1.1 mm Hg/ml, P less than .01), while the heart rate increased (109 +/- 15 vs. 118 +/- 16 min-1, P less than .05). There were no significant changes in LV end-systolic volume, stroke volume or the maximum time derivative of LV pressure. The plasma felodipine concentration was 16.1 +/- 1.4 nmol/l (mean +/- time S.D.). Three relations that provide load-insensitive measures of LV performance were determined from variably loaded pressure-volume loops produced by transient caval occlusions. Felodipine increased the slopes of the LV end-systolic pressure-volume relation (8.1 +/- 0.9 vs. 10.3 +/- 1.3 mm Hg/ml, mean +/- S.D., P less than .05), the maximal time-derivative of LV pressure-end-diastolic volume relation (103 +/- 28 vs. 127 +/- 34 mm Hg/sec ml, P less than .05) and the stroke work-end diastolic volume relation (82 +/- 4 vs. 93 +/- 6 mm Hg, P less than .05). All three relations were shifted toward the left after felodipine. Similar increases in slopes and leftward shifts with felodipine, indicating enhanced contractile effect, were also present after autonomic blockade.(ABSTRACT TRUNCATED AT 250 WORDS)