Summary. In Exp. I oxytocin (60 μg/100 kg/day) was infused into the jugular vein of 3 heifers on Days 14–22, 15–18 and 16–19 of the oestrous cycle respectively. In Exp. II 5 heifers were infused with 12 μg oxytocin/100 kg/day from Day 15 of the oestrous cycle until clear signs of oestrus. Blood samples were taken from the contralateral jugular vein at 2-h intervals from the start of the infusion. The oestrous cycle before and after treatment served as the controls for each animal. Blood samples were taken less frequently during the control cycles. In Exp. III 3 heifers were infused with 12 pg oxytocin/100 kg/day for 50 h before expected oestrus and slaughtered 30–40 min after the end of infusion for determination of oxytocin receptor amounts in the endometrium. Three other heifers slaughtered at the same days of the cycle served as controls.
Peripheral concentrations of oxytocin during infusion ranged between 155 and 641 pg/ml in Exp. I and 18 and 25 pg/ml in Exp. II. In 4 out of 8 heifers of Exps I and II, one high pulse of 15-keto-13,14-dihydro-prostaglandin F-2α (PGFM) appeared soon after the start of oxytocin infusion followed by some irregular pulses. The first PGFM pulse was accompanied by a transient (10–14 h) decrease of blood progesterone concentration. High regular pulses of PGFM in all heifers examined were measured between Days 17 and 19 during spontaneous luteolysis. No change in length of the oestrous cycle or secretion patterns of progesterone, PGFM and LH was observed. The number of oxytocin receptors in endometrium was not affected by oxytocin infusion around the time of oestrus. These results suggest that luteolytic events were not significantly influenced by a constant infusion of oxytocin.