The stimulus-response characteristics of cardiopulmonary baroreflex control of forearm vascular resistance (FVR) were studied in five unfit [UF, maximal O2 consumption (VO2 max) = 38.5 ml X min-1 X kg-1] and six fit (F, VO2 max = 57.0 ml X min-1 X kg-1) subjects. We assessed the relationship between reflex stimulus, i.e., changes in central venous pressure (CVP) and response, i.e., FVR, during selective unloading of the cardiopulmonary mechanoreceptors with lower body negative pressure (0 to -20 mmHg). The linear relationship between FVR and CVP, the gain of this baroreflex, was significantly diminished in the F subjects, -2.42 +/- 0.57 U/mmHg, compared with the UF, -5.15 +/- 0.58 U/mmHg. Both groups, F and UF, had similar resting values for CVP and FVR; thus the diminished gain in F subjects was not simply an artifact resulting from a shift of the set point along the baroreflex stimulus-response curve. We also found a linear relationship between baroreflex gain and total blood volume (r = 0.59, P less than 0.05). We conclude that the gain of this vascular reflex is attenuated in trained individuals and is related to cardiovascular adaptations, such as an increased blood volume, associated with exercise training.