The effect of prolonged continuous hypoxia with polycythaemia on the rabbit glomerulus was studied by light microscopy and their effect on experimental serum sickness in rabbits was studied by light and immunofluorescent microscopy. Engorgement and enlargement were the principal effects on the glomerulus of hypoxia and polycythaemia, but prominence of the mesangium and focal segmental sclerosis and hypercellularity sometimes occurred. The results of the studies on serum sickness suggested that prolonged hypoxia with polycythaemia did not diminish antibody formation, reactivity of antibody with antigen, or rapid antigen clearance; in fact antigen was cleared more rapidly in hypoxic animals. Chronic hypoxia and polycythaemia did not prevent acute serum sickness including acute glomerulonephritis. The incidence and severity of acute glomerulonephritis, however, may have been diminished whereas the incidence and severity of other visceral involvement by acute serum sickness was not. Although the experimental groups were not large, the possible implications of the data are important. Focal segmental glomerular sclerosis and hypercellularity, and mesangial prominence, to our knowledge have not previously been reported in association with chronic experimental hypoxia and polycythaemia. These lesions resembled some glomerular lesions of cyanotic congenital heart disease. Decrease in glomerulonephritis under conditions in which hydrostatic pressure in the glomerulus is presumably elevated and the glomerulus is subject to injury in some respects would be contrary to expectation. Finally, selective decrease in the glomerulonephritic but not the other lesions of acute serum sickness by altering non-immunological general experimental conditions would be highly unusual. More extensive studies are indicated.