This study evaluated the feasibility of using the modified semi-closed circuit underwater rebreathing system (URS) LAR VII in connection with a laryngeal mask airway (LMA) as an expedient ventilatory adjunct in an operational setting. Fourteen combat swimmers, unfamiliar with this equipment, underwent cardiopulmonary resuscitation (CPR) mannequin training using these devices. Eighteen subjects, using a standard AMBU bag for ventilation, served as controls. Thirteen test persons were able to ventilate with the modified URS. Tidal volumes were significantly lower with the LAR VII/LMA than with the AMBU bag (medians, 350 vs. 800 mL). No significant difference was found in total time required for 10 CPR cycles (medians, 78 vs. 68.5 seconds). The median delay between recognition of cardiac arrest and first chest compression, however, was markedly increased with the LAR VII/LMA than with the AMBU bag (medians, 76.5 vs. 14.5 seconds). After proper training, divers might use a modified URS such as the LAR VII for CPR in connection with a LMA. Lower tidal volumes might prevent gastric inflation. Chest compression should be continued during LMA insertion.