During hysteroscopy the uterus may be distended with carbon dioxide (CO2), nitrous oxide (N2O), or Hyskon (a high molecular weight dextran). An initial study in 27 patients (group 1) using arterialized venous blood samples demonstrated rises in carbon dioxide tension (PCO2) when N2O was insufflated by using a laparoscopy insufflating device--a constant-pressure, variable-volume gas source. Cardiovascular collapse occurred in one patient in this group, most probably as a result of macropulmonary emboli of N2O. The rise in PCO2 is accounted for by an increase in physiologic dead space. In another 24 patients (group 2) the gaseous media were introduced by using a constant-volume, variable-pressure gas source; this resulted in minimal changes in arterial PCO2. The choice of whether a gaseous or liquid distending medium is used for hysteroscopy is governed by the state of the endometrium. If a gaseous medium is indicated, then CO2 is preferable to N2O and should be introduced with a constant-volume, variable-pressure gas source.