Recent studies in normal participants have shown that right to left shunt blood vessels in the lung open up during exercise. We describe the first field study to investigate this phenomenon at altitude. This study aimed to assess the effect of altitude and partial acclimatization on inducible right to left shunting at rest and with exercise. A contrast-enhanced transcranial Doppler imaging technique to detect microbubbles after injection of blood and saline agitated with air was used to measure right to left shunting in 10 normal participants at rest and immediately after exercising to maximum oxygen consumption (VO(2max)) at 80 m, on acute exposure to 3450 m, and finally after a week above 3450 m. At 80 m, exercising resulted in right to left shunting via patent foramen ovale in 2 participants, but there was no evidence of shunting in the remaining 8 participants. Cerebral microbubbles were detected at rest in the 2 participants with patent foramen ovale on acute exposure to 3450 m, and the shunting increased on exercise (P = .04). In 5 of the remaining 8 participants without patent foramen ovale, cerebral microbubbles were detected on exercise (P = .04) but not at rest. Partial acclimatization had minimal effect on the prevalence or magnitude of the intrapulmonary or intracardiac shunts. Oxygenation was similar in those with shunts compared with those without shunts. Intrapulmonary shunting occurs on exercise at altitude, but the clinical and physiologic significances have yet to be determined. Despite the occurrence of shunting in most participants, our results suggest that this phenomenon is not a significant factor in altitude and exercise-induced hypoxia.