The effects on patient oxygenation of nitrous oxide, narcotic and epidural analgesia in labour were evaluated using pulse oximetry. Five groups of ten patients received either no analgesia (Control, Group 1), an epidural block alone (Group 2), nitrous oxide in oxygen alone (Group 3), intramuscular pethidine (Group 4), or nitrous oxide in oxygen combined with intramuscular pethidine (Group 5). Derived parameters included the maximum (MAX), minimum (MIN), average maximum (AV MAX), and average minimum (AV MIN) arterial haemoglobin oxygen saturation (SaO2), and differences between maxima and minima (MAX-MIN). There was a statistically significant difference between Group 1 and Group 5 for MIN, AV MIN and MAX-MIN SaO2 (P less than 0.05). All other groups showed no significant difference in any parameter when compared with the control group. The results are discussed with reference to normal and disordered maternal physiological changes in pregnancy. It is suggested that nitrous oxide should not be used for analgesia in labour where there is concern about maternal, placental or foetal reserve.