Myometrial activity is not completely inhibited during pregnancy in the guineapig, but it is reduced to a level significantly below that of the non-pregnant animal (Porter, 1971). In view of a recent finding (Porter, 1969, 1970) that progesterone was totally without effect on myometrial activity in vivo in the pregnant and non-pregnant guinea-pig, it seemed possible that relaxin might be implicated in the mechanism by which myometrial activity is reduced in pregnancy in this species. This hormone increases in concentration in the serum during pregnancy in guinea-pig (Zarrow, 1947) and inhibits guinea-pig uterine contractions in vitro (Felton, Frieden & Bryant, 1953). A study was carried out therefore on the effects of the hormone in vivo.
Non-pregnant guinea-pigs in various stages of the oestrous cycle were equipped with intrauterine recording balloons according to a method described earlier