The effects of sodium dantrolene (DaNa), a calcium antagonist specifically acting on skeletal muscles, were evaluated on the spontaneous contractions of ileum, uterine horns and longitudinal strips of esophageal smooth muscle, in vitro. DaNa (1.81 to 15 μg/ml) caused a reversible dose-related increase of the spontaneous contractions of the three organs. DaNa (15 μg/ml) caused a 60±3% increase of the ileum spontaneous tone, and caused a 57±3% increase in the frequency of the spontaneous contractions of the uterus. The stimulatory effect of DaNa on the uterine musculature was not antagonized by atropine, methysergide or indomethacin. In the esophagus the maximum increase of the spontaneous tone and contractions (65±2%) was due to 3.7 μg/ml perfusion of DaNa. This effect was antagonized by nifedipin
