In mothers nursed in a semi-recumbent position, arterial oxygen tension was measured at the beginning and END OF AN an intermittent period of inhalation, Lasting for six contractions, during the first stage of labour. Ten mothers had methoxyflurane 0.35 per cent and air, followed by methoxyflurane 0.35 per cent with approximately 50 per cent oxygen. Another 10 mothers had 50 per cent nitrous oxide and 50 per cent oxygen (Entonox). The mean values of PaO2 at the beginning of periods of inhalation in those who received methoxyflurane in air, methoxyflurane in 50 per cent oxygen, and nitrous oxide in oxygen were 109, 120 and 106 mm Hg respectively. The mean increases in PaO2 by the ends of periods of inhalation were 18, 43 and 63 mm Hg respectively. On average, increasing the inspired concentration of oxygen to 50 per cent significantly and substantially increased maternal PsO2 with both agents by the emd pf inhalation. There would therefore be some increase in available oxygen during the phase of higher blood flow following a uterine contraction. However, the lowest maternal PaO2 measured was 72 mm Hg; therefore additional oxygen is unlikely to offer much advantage except in patients with respiratory or cardiac deficiencies. In the intervals between inhalations of the analgesic mixture the PaO2 decreased to about the level found during air breathing. Consequently it would be necessary to breather additional oxygen continuously, between as well as during contractions, to ensure an increased maternal PaO2 throughout labour.