The effects of isoxsuprine on the myometrium were examinedin vitroin 12 uterine strips of 6 rabbits andin situin 12 rats and 20 dysmenorrheic patients.In vitro,isoxsuprine reduced isometric tension in both electrically stimulated and spontaneously contracting uteri. At 30 minutes after treatment, 200 μg/ml isoxsuprine suppressed tension by 60% (P< 0.001). In unanesthetized rats, 500 μg isoxsuprine reduced the intrauterine pressure (IUP) by 37% (P< 0.001), and 1000 μg by 45% (P< 0.001), during 76 ± 14 and 100 ± 4 minutes, respectively, The frequency of IUP cycles was also reduced significantly (P< 0.001). In menstruating patients, 60 mg isoxsuprine reduced the IUP during 2 hours by 46% (P< 0.001) and decreased the subjective symptoms of pain (P< 0.001). The frequency of IUP cycles was also reduced significantly (P< 0.001). The suppression of IUP was explained by the reduction of wall tension (WT), since IUP = 2WT/R (where WT = uterine wall tension and R = radius). This combination ofin vitroandin situstudies in “animal models” and patients is recommended for the quantitation of the therapeutic actions of drugs.