Pre-oxygenation times were studied in 30 fit pregnant women during the course of their pregnancy. The mothers were studied in the supine position with a left lateral tilt and breathing through a standard Magill system with an oxygen flow rate of 8 litres/minute. Between 13-26 weeks gestation and 26-42 weeks the times needed to achieve an end-tidal nitrogen of 2% were 104 seconds (SD 30) and 80 seconds (SD 20), respectively. A control group of young women of similar age and weight needed 130 seconds (SD 30) to achieve the same end-tidal nitrogen concentration. A gas-tight mask was found to be essential to achieve effective denitrogenation (end-tidal nitrogen concentration of 2%).