Pulmonary arterial (PA) and pulmonary capillary (PC) pressures and oxygen saturation were measured over a 5-minute period on immersion in water and during swimming in four symptom-free men (aged 52-58 years) who had sustained a transmural infarct 6-10 weeks previously. Measurements were made through an indwelling flow-guided percutaneously introduced catheter. The speed of swimming was 20-25 m/min. Static pressures on immersion up to the neck rose up to 23 mm Hg, on diving to five times as much above the resting recumbent value. Mean PC values after five minutes of swimming rose in all four subjects above those obtained after a 5-minute 100 W exercise: they were similar to the diastolic pressures. But the oxygen saturations on swimming were in all patients higher than after tread-wheel exercise. All patients felt swimming to be easier than the ergometric exercise. It is concluded that noninvasive methods, such as X-rays, ergometry and echocardiography, are not sufficient for estimating volume stress induced by immersion in water. Cardiac arrhythmias did not occur.