Paracervical block (PCB) given during labour reduces the uterine activity necessary for cervical dilatation. The aim of the present investigation was to find out whether this effect of PCB changes the form of the amniotic pressure curve. A preliminary investigation showed that the ascending limb of the amniotic pressure curve, A, consists of three phases. The first phase shows an increasing slope and is called the acceleration phase of the ascending limb, Aa; the second phase is rectilinear, Ar. The third phase has a decreasing slope, and is named the deceleration phase, Ad. The descending limb of the amniotic pressure curve, D, was found to have corresponding phases. The initial part has a downward increasing slope and is called the acceleration phase of the descending limb, Da. Then follows a rectilinear phase, Dr and finally a phase with a decreasing slope, the deceleration phase, Dd. The duration of each phase, except Dd, was measured during an observation and a treatment period in 27 patients receiving PCB, and 25 control cases who were given N2O/O2, pethidine, or a combination of both analgesics. The patients were strictly selected and allocated at random to either group. Local analgesia in combination with adrenaline was given at cervical dilatation 3-6 cm; 20 minutes were allowed to elapse from the administration of analgesia until registration of the contraction of the treatment period, to assure an established analgetic effect. PBC shortens Aa and Ad, but prolongs Ar. A + Da + Dr and Ad + Da are shortened while Dr is prolonged after PCB (Table IV). On the presumption that PCB inactivates a part of the lower uterine segment, the changes of Aa, Ar and Ad + Da were predictable. To determine the significance of adrenaline, another series of 21 patients were given PCB, injecting 0.25 per cent bupivacaine without adrenaline added to the solution. The duration of the phases before and after the block was compared. PCB without adrenaline brought about the same changes as it did with adrenaline, but the changes were smaller, only those of Aa, Ar, Dr and Ad + Da being statistically significant. The pressure curves of six patients receiving epidural block were examined using the same method as in the PCB and control groups. The changes typical of PCB did not occur after epidural block. The discovery of the six phases of the amniotic pressure curve, their change after PCB and the results of other investigators make possible a suggestion of the basic physiological processes that influence amniotic pressure during coordinated labour contractions. It is held that the form of the amniotic pressure curve can be explained in terms of contraction, relaxation, and propagation of a contraction and a relaxation wave.