Breathing In, Breathing Out was performed twice, in Belgrade (1977) and Amsterdam (1978). For this performance the two artists, Abramović and Ulay, blocked their nostrils with cigarette filters and pressed their mouths together, so that one couldn’t inhale anything else but the exhalation of the other. As the carbon dioxide filled their lungs, they began to sweat, move vehemently and wear themselves out; the viewers could sense their agony through the projected sound of breathing, which was augmented via microphones attached to their chests. It took them nineteen minutes in the first performance and fifteen in the second to consume all the oxygen in that one breath and reach the verge of passing out.