Analogues of oxytocin contract the rat uterus in vitro more strongly in the presence of 0·5 mm-Mg2+ than in the absence of the ion. This effect can be referred to an increased 'affinity' of the hormone for its receptors. Neither Ca2+ (1 mm), Sr2+ (0·5 mm) nor lower concentrations of Mg2+ (0·1 mm) have this effect. Manganese mm) was more effective than Mg2+ (0·5 mm); the former potentiated the action of 3-valine oxytocin 9·9 times; that of 8-arginine oxytocin 3·3 times and that of 8-isoleucine oxytocin 1·3 times while 0·5 mm-Mg2+ potentiated the same analogues 5·7, 1·9 and 1·1 times, respectively. The results are discussed in relation to the mechanism of action of oxytocin.