SUMMARY

The guinea-pig uterus and strips of human myometrium are stimulated in vitro by ether extracts of menstrual fluid; the guinea-pig uterus is also stimulated in vivo by intravenous or intra-cardiac injections of the same extracts. The amount of the active material recovered from menstrual fluid probably varies from subject to subject, and in one instance was shown to decrease steadily from the beginning to the end of the menstrual period. The uterine responses in vitro take several forms, all excitatory; they are compared with the contractions of the human uterus in vivo during normal menstruation and dysmenorrhoea. Among the lipids of the human endometrium are a group of slowly acting stimulants of plain muscle. There is some evidence that these or similar substances circulate in the systemic blood during menstruation, in concentrations great enough to stimulate the myometrium; and such an action has been suggested as the main physiological function of these stimulants [Pickles, 1957, 1959; Chambers & Pickles, 1958]. The present paper describes the uterine responses in detail, and gives some account of the variations in the amounts of the stimulants recovered from the menstrual fluid. The substances concerned are not yet identified and are referred t