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Uterine electromyography was performed by means of chronically implanted surface electrodes in 3 Pony mares during spontaneous oestrous cycles and following luteolysis induced by a prostaglandin analogue (fluprostenol). Three distinct patterns were recognized during the oestrous cycle. (1) During oestrus well defined phases of activity with closely grouped high-amplitude spikes were separated by long periods (10-45 min) of complete inactivity. (2) During dioestrus more diffuse phases of activity with low-amplitude spikes were separated by variable periods of relative inactivity. (3) During luteolysis, short and frequently occurring phases of activity were propagated between the two electrodes on one uterine horn; a similar pattern also occurred between 1 and 3 h after injection of fluprostenol. Peripheral plasma progesterone, but not total inconjugated oestrogen, concentrations were closely related to characteristics of the myographic activity during the cycle. Insemination during oestrus and injection of fluprostenol during dioestrus caused a marked and prolonged increase in myometrial electrical activity. Almost any non-specific environmental stimulus, including entry by palpation of the genital tract per rectum and vaginoscopic examination, but these were of brief duration and the normal resting pattern of activity was quickly re-established after completion of the manipulations.

Summary Levels of the major circulating metabolite of prostaglandin F‐2 α, 13, 14 dihydro‐15‐oxo‐prostaglandin F‐2 α (PGFM) were measured during the induction of foaling using small (2.5–10 iu) intravenous doses of oxytocin. PGFM levels rose rapidly in all animals within 15 min of injection and were associated with typical signs of second stage labour. Because these small doses of oxytocin are effective in successfully triggering parturition it is suggested that higher doses (40–120 iu) used to induce birth in other studies are unnecessary and could be potentially dangerous to the foetal foal. Résumé Les taux des principaux métabolites circulant de la prostaglandine F‐2α, 13, 14 dihydro‐15‐oxo‐prostaglandin (F‐2α PGFM) ont été mesurés durant l'induction du poulinage utilisant de faibles doses (2, 5–10 iu) d'ocytocine intraveineuse. Les taux de PGFM s'élevèrent promptement chez tous les animaux dans les 15 minutes suivant l'injection et furent simultanés de signes cliniques de parturition. Puisque ces petites doses d'ocytocine sont efficaces dans le déclenchement de la mise bas, on pense que les posologies plus élevées (40–120 iu) utilisées dans d'autres études pour induire le poulinage sont inutiles et qu'elles pourraient représenter un risque pour le foetus. Zusammenfassung Man mass die Konzentrationen des hauptsächlichen zirkulierenden Metaboliten von Prostaglandin F‐2×13, 14 Dihydro 15‐oxo‐prostaglandin F‐2X (PGFM)—während der Geburtseinleitung mit niedrigen intravenösen Dosen von Oxytocin (2.5–10 iu). Die PGFM‐Spiegel stiegen rasch innert 15 Minuten bei allen Tieren an und waren mit typischen Zeichen des 2. Wehenstadiums. Weil diese niedrige Oxytocindosen wirksam und erfolgreich verbunden die Geburt auslösen, wird vermutet, dass höhere Dosen (40–120 iu) zur Geburtseinleitung unnötig oder sogar potentiell gefährlich für das Fohlen seien.