Total urine portions were collected from 31 pilots and candidates for pilots before hypobarism, during 35-40 minutes of hypobarism at a simulated altitude of 5000 m (540 hPa) and following hypobarism. The measured volume of urine portions and amounts of glycosaminoglycans (GAG) and hydroxyproline (Hp) were calculated in 30-minute periods before the test, during hypobaric test and following this test. The average urine volume during hypobarism and average amounts of GAG and Hp in the urine collected during this period were found to be significantly increased in comparison with similar quantities before hypobarism. After the hypobarism the average urine volume was still augmented, whereas the average amount of Hp was lower than before hypobarism. During hypobarism the volume of urine and the amount of GAG and Hp were significantly positively correlated.