Cardiovascular response to supine exercise was studied in 11 patients with chronic ventricular pacing for complete atrioventricular block whose heart rate was fixed during exercise, and compared with 11 age- and sex-matched normal subjects. Oxygen uptake increased linearly with increasing work rate, and attained maximum values of 15.7 +/- 0.7 and 29.5 +/- 1.1 ml/min.kg, respectively. In both groups, cardiac output increased in association with oxygen uptake, although the maximum value in the patients was less than one half that in the controls (6.8 +/- 0.5 and 15.1 +/- 0.3 l/min). Stroke volume, however, could change in patients, as in the controls under similarly increased afterloads (maximum values were 96 +/- 7 and 93 +/- 3 ml/beat, respectively). During exercise, the systolic arterial pressure elevated to a similar extent in both groups (221 +/- 14 and 235 +/- 12 mmHg, respectively). The increase in stroke volume in the patients was achieved by complete systolic emptying and the Frank-Starling mechanism. It is considered that in patients with ventricular pacing, exercise can produce an increase in stroke volume when myocardial function is not impaired.