The effect of prostaglandin (PG)-induced luteolysis on the myometrial activity in 20\p=n-\21-day-pregnant and 11\p=n-\12-day-pseudopregnant rabbits was studied by intrauterine pressure (IUP) recording during PG infusions. The same dose of PG (10 \g=m\g/hduring 8 h) was also given to 7 non-pregnant (untreated) does that were used as controls. Peripheral plasma concentration of progesterone and oestradiol-17\g=b\ were measured at 2-h intervals during the infusion. Plasma progesterone level

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... ed significantly within 2 h of the start of infusion in pregnant and pseudopregnant does and continued to decrease; at the end of 8 h, the concentrations were 31 and 41% respectively, of the pre-infusion levels. The amplitude of uterine contractions increased significantly after 4 h in pseudopregnant does, increased slightly but insignificantly in the pregnant does and showed no significant change in the non-pregnant does during PG infusion. The amplitudes developed in the pregnant and pseudopregnant does were significantly different. The direct effect of progesterone (1\p=n-\3\g=m\g/hduring 4 h) was also studied in 7 non-pregnant rabbits. After 2 h the amplitude of contractions had decreased markedly and the pattern of activity had become irregular. The results support the concept of a myometrial inhibitory factor other than progesterone in rabbit pregnancy and suggest that this factor(s) originates in the conceptus.