Urinary protein excretion was measured before and after the intravenous infusion of lysine in 14 normal subjects after 4-6 days' acclimatization at 4846 m. Urinary albumin excretion before lysine was elevated in 11 subjects but alpha 1-microglobulin was detected in only four. After lysine a large increase in albumin excretion occurred in all subjects. Together with the absence of alpha 1-microglobulin before lysine this implies that increased glomerular capillary permeability is the major cause of proteinuria after acclimatization to high altitude. The estimated minimum glomerular fluid albumin concentration was increased two to three fold above the published values in normal controls.