Propranolol (1; Obsidan) relaxes in vitro stripes of smooth muscles of non-pregnant rat's uterus in a concentration dependent manner. In contrast, on uterine muscle stripes of pregnant rats 1 causes small contractions. In an combination of 1 and fenoterol (2; Partusisten) (20:1) 1 prevents the relaxing effect of 2, reported to be an inhibitor of uterine muscle activity. This antagonism is most distinct on stripes of pregnant rat's uterus.