To investigate the clinical effect of SR49059 when given shortly before the onset of menstruation as a preventative treatment of dysmenorrhoea. A double-blind, randomised, placebo-controlled, cross-over trial in complete block design (three periods, three treatments). A clinical research organisation in Paris, France. Women aged 18-35 years suffering from primary dysmenorrhoea. In each of three menstrual cycles, women reported to the study centre and were given a daily dose of either placebo, 100 mg or 300 mg SR49059 from a minimum of 4 hours up to a maximum of three days before the onset of bleeding and/or menstrual pain. If this did not control the pain, women were allowed once a day to take a second dose of study treatment providing that at least 4 hours had passed since the first drug intake. Intensity of menstrual pain recorded by means of a visual analogue scale. Rating of symptoms of dysmenorrhoea (mainly back and pelvic pain) in relation to functional capacity (Sultan score). Self-assessment of menstrual blood loss in a menstrual diary record. Analysis of intensity of menstrual pain, as recorded by visual analogue scale and Sultan pain score (back and pelvic pain) during the first 24 hours of dysmenorrhoea, showed a dose-related effect of SR49059. The 300 mg dose of SR49059 was significantly more effective than placebo. Similarly, a dose-related effect of SR49059 was shown on total Sultan score. SR49059 was well tolerated and no significant effect on the bleeding pattern was noted. This study showed for the first time a therapeutic effect of an orally active vasopressin V1a receptor antagonist in the prevention of dysmenorrhoea. Further studies are required to examine effect mechanisms and determine effective doses.