The arterial oxygen saturation of 40 mothers in the first stage of labour was monitored using pulse oximetry. Half the mothers received epidural analgesia and the rest inhaled Entonox for pain relief. Eight mothers in the Entonox group and six in the epidural group had at least one episode of significant hypoxia (saturation < 90%). There was little difference in the number of hypoxic episodes experienced by either group (29 in the Entonox and 21 in the epidural) although their mean duration and severity was greater in the Entonox group. Women in labour who inhale Entonox have an appreciable incidence of arterial desaturation. Epidural analgesia reduces the severity of hypoxic episodes although it does not eliminate them.