A technique for complete oxytocinase inhibition has been combined with a rapid serial sampling strategy to determine the plasma oxytocin concentration in eight women undergoing amniotomy for the induction of labour at term and in eight control women. Amniotomy was not associated with an increase in plasma oxytocin concentration. Furthermore, the concentrations were not increased for 20 min following the procedure. Labour was not established in any patient by the end of the 30-min study period.