Mechanisms of possible pathophysiological importance in primary dysmenorrhea are discussed. Hyperactivity of the myometrium with accompanying uterine ischemia is considered to be of central importance in the causation of pain. Prostaglandins seem to be involved to a large extent in the development of the myometrial hyperactivity. Other mechanisms of possible importance such as ovarian hormones, cervical factors, vasopressin, nerves, and psychological factors can well act ultimately through prostaglandin release but an action directly on the myometrium and blood flow may also occur.