In 76 women with spontaneous labour or labour induced with oxytocin or PGF2alpha, intra-uterine pressure was recorded during the first stage of labour. A statistical analysis was performed of the intensity and frequency of contractions, the total uterine activity and the variability of peak-to-peak intervals and amplitudes of contractions. The duration of the latent phase was shortened by 50% in induced labour compared with spontaneous, while the active phase was less affected. However, the intensity and frequency of contractions and the uterine activity in Montevideo Units were of the same magnitude in spontaneous as in induced labour, apart from two cases of hyperactivity found in the induction groups. The variability of intervals between contractions and amplitudes of contractions were also of the same degree in all groups and no overall tendency towards greater regularity was seen during progress of labour. If irregular intervals between contractions were noted, when active cervical dilatation had begun, the progress of labour was slower and more uterine work was needed for dilatation than when the uterine activity was more regular.