1. Progressive exercise and circulatory studies are reported on a selected group of patients with hypoxaemia and secondary polycythaemia before and after therapeutic decrease of packed cell volume.
2. A significant increase in exercise tolerance was demonstrated in a group of seven patients who claimed subjective benefit from the treatment.
3. No important circulatory changes were detected at rest after treatment.
4. During steady exercise mean pulmonary artery pressure at any given cardiac out-put was lower after treatment suggesting a fall in pulmonary vascular resistance.
5. During steady exercise oxygen consumption was increased in five of six patients subjectively improved by the treatment but no consistent changes in arterial lactate concentration or lactate/pyruvate ratio were observed.