The effect of laparotomy on the respiration is studied by measuring its effect on the various factors concerned in respiratory activity. 64 patients were studied before, and from day to day after, operation. The first 5 days after operation were studied the most intensively, since it is during this period the majority of postoperative pulmonary complications develop. 4000 values are presented graphically. Standard errors are given. It is shown that laparotomy profoundly affects the respiration, producing: (1) marked reduction in tidal air. (2) marked increase in respiratory rate. (3) a slight, not significant, change in total ventilation. (4) rapid, shallow respiration which sets up a vicious cycle tending to increase further the rate. (5) great reduction in complemental air, greater following upper than lower abdominal operations in both men and women. (6) great reduction in supplemental air, greater following upper than lower abdominal operations in men but not in women. (7) about the same degree of crippling of the mechanism of forced inspiration and of forced expiration. (8) great reduction in vital capacity, the greater crippling in both sexes following upper abdominal operations and greater crippling in men than in women.