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SUMMARY1. Prostaglandin E1 or E2 has two clearly distinguishable effects on the guinea-pig myometrium in vitro.2. The first of these, the 'direct' effect, is manifested by contraction accompanied by spiking, is readily modified by changes in the concentrations of Mg2+, Ca2+ and K+ in the suspension medium, and does not long outlast the presence of the prostaglandin in the organ-bath. In this respect it resembles the responses to other stimulants.3. The second effect is manifested by 'enhancement' of the responses to non-specific stimulation, whether this be by chemical or electrical means. Enhancement may be demonstrated for up to 20-80 min after the PGE1 or PGE2 has been washed out of the organ-bath, and during this time the myometrium shows no consistent changes in electrical activity or in resting mechanical tension that may be associated with the enhancement effect. Enhancement following PGEs is not readily affected by moderate changes in the Mg2+, Ca2+ or K+ concentrations of the suspension medium, and in all these respects it differs markedly from the effects of other stimulants.4. K+-depolarized myometrial preparations show feeble direct responses to PGEs, like those to other stimulants. They also show post-

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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 intracardiac 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.