Preterm delivery is the largest cause of perinatal mortality and morbidity, yet the treatment of preterm labour has not been demonstrated to improve outcome. The reasons are numerous and complex, but they include a failure to understand the mechanism(s) of preterm labour, the multitude of different causes, the difficulty in diagnosis and the problems of outcome measurement in clinical trials. Recently, an oxytocin antagonist (atosiban) has been introduced into clinical practice in Europe. Although it may be an effective tocolytic, a beneficial effect on perinatal outcome has not been demonstrated. Atosiban has an effect at both oxytocin and vasopressin (V1a) receptors, which (assuming efficacy) raises the question as to whether oxytocin or vasopressin V1a antagonism is required for tocolysis. This review examines the rationale for tocolysis in preterm labour, the evidence for administration of atosiban and the role for oxytocin, vasopressin and their receptors in the onset of labour. Experimental Physiology (2001) 86.2, 297-302.