The uterus-relaxant effect of diltiazem, a recently developed calcium antagonist, was investigated in vitro. 24 myometrial strips dissected from the lower uterine segment during cesarean section showed a statistically highly significant (p < 0.001) decrease of their spontaneous contractility in the presence of Diltiazem 10––6M: 82% after 10 min, 100% after 20 and 100% after 60 min (median values). Diltiazem, which is considered to be the most balanced calcium antagonist as far as cardiovascular side effects are concerned, could be of considerable interest also for clinical use.