To assess whether acute alveolar hypoxia leads to the release of endothelin-1 (ET) in vivo, ET, cortisol (CORT), and lactate (LA) levels were determined in 15 healthy subjects at rest and during exhaustive incremental cycle ergometry on two separate test days. The subjects were to breathe a gas mixture with reduced O2 content ([H] fraction of inspired O2, 0.14) on one day and normal air on the other (N). Modified responses of LA and CORT to exhaustive incremental cycle ergometry on the H day indicated elevated anaerobic tissue metabolism and increased physical stress. With acute alveolar hypoxia, the response of ET to exhaustive physical labor was found to be augmented.