Central and peripheral cardiovascular adaptations to 12 weeks of intense swim training were characterized in 12 previously sedentary middle-aged men and women. Peak oxygen uptake (V̇O2) during upright bicycle exercise improved from 29.2 ± 5.6 to 34.7 ± 6.7 ml/kg/min (mean ± SD, p < .01) because of similar increases in peak cardiac output (CO) and calculated arteriovenous oxygen difference (both p = .02). Peak supine V̇O2 was 10% higher after training (p < .005) solely because of enhanced CO (p = .005). Peak heart rate decreased in both postures; therefore stroke volume at peak exercise was greater by 10% and 18% in the upright and supine postures, respectively (p = .05 and p = .005). There was an identical 18% rise (p = .01) in peak supine left ventricular end-diastolic volume index by radionuclide ventriculography but no change in left ventricular ejection fraction or end-systolic volume index (ESVI). Peak systolic blood pressure (SBP) was unchanged in the upright posture but was 8% higher (p = .002) during recumbency despite a similar total peripheral resistance and SBP/ESVI ratio. Maximal calf conductance (G(max)), assessed separately by venous occlusion plethysmography after local ischemic exercise to fatigue, was augmented 20% (p < .02) by training, resulting in an 18% greater hyperemic blood flow (p = .05). Peak V̇O2, CO, and G(max) were unchanged in five nonexercising control subjects. We conclude that in middle-aged humans, intense swim training for 12 weeks produces adaptations that include a greater capacity for vasodilatation in skeletal muscle and an enhanced cardiac pump capacity.