Gas exchange occurring in the abdominal cavity during laparoscopy, using carbon dioxide as the insufflating gas, was investigated in 25 female patients being ventilated with 66.6% nitrous oxide and 33.3% oxygen. The gas remaining in the abdomen at the end of the procedure was collected and measurements were made using an infrared spectrometer, a paramagnetic analyser and a mass spectrometer. The mean duration of the laparoscopy was 9.5 minutes and the mean volume of carbon dioxide delivered was 6.8 litres. Nitrous oxide concentration in the abdomen was found to increase significantly with the duration of the procedure, varying from 1.4% to 12.8% with a mean of 4.3% (s.d.±2.4). Oxygen concentration measured from 0.1 to 1.8% with a mean of 0.7% (s.d.±0.4). Nitrogen concentration varied from zero to 1.8%, having a mean concentration of 0.8% (s.d.±0.5). Carbon dioxide content was from 85.7 to 99.6% with a mean concentration of 94.2% (s.d.±3.1).