Entonox (50% nitrous oxide in oxygen) and isoflurane (0.75% in oxygen) were compared as analgesics in the first stage of labour in 32 consenting women. The drugs were self-administered and given in random sequence, each during five consecutive uterine contractions. Each patient acted as her own control. Linear analogue pain scores were significantly lower (p less than 0.001) with isoflurane than with Entonox, but scores for drowsiness were higher for isoflurane. Further study is needed to assess the effects of more prolonged use of isoflurane in labour.