The method used enables one to record directly the pressure-volume diagram of a subject while breathing against a high resistance. The average work accomplished during expiration was 69% and, during inspiration, 79% of that which was predicted from the pressures developed against a column of mercury at different lung volumes. When maximal respiratory efforts are assessed, one can expect to incur the same problems of defining maximum as are encountered with other muscles. Submitted on June 2, 1960