The value of oxygen in conjunction with the roentgen ray as an aid in the diagnosis of obscure abdominal conditions has been demonstrated in a number of recent publications. No ill effects have accompanied or followed the pneumoperitoneum produced by inflation of the abdominal cavity with oxygen gas. The tolerance of the peritoneum for oxygen even in large volume, and the fact that its presence can be detected by fluoroscopy and roentgenography have led to its use as a diagnostic procedure in determining patency of the fallopian tubes. If the gas injected into the uterus under certain measurable pressure would pass into the fallopian tubes, it ought to reach the general peritoneal cavity. In patients with patent fallopian tubes the gas would establish an artificial pneumoperitoneum identical with that produced when injected by direct abdominal puncture. In patients with occluded tubes no such result could be obtained. Accordingly, experiments were