Active immunization against oxytocin significantly prolonged the oestrous cycle in 3 out of 4 goats; the mean (\m=+-\s.e.m.) cycle length was 29\m=.\1\ m=+-\1\m=.\7days (n = 12) compared to 19\m=.\4\ m=+-\0\m=.\6 days (n = 9) in control animals. During Days 10\p=n-\21of the cycle in the 3 responsive goats, peripheral plasma concentrations of progesterone and oxytocin were steady and those of 13,14-dihydro-15-keto-prostaglandin F-2\g=a\ were very low (50\p=n-\100pg\m=.\ml \m=-\1) with no marked

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... ulsatile activity. The major effect of immunization would appear to be suppression of the synthesis of the uterine luteolysin PGF-2\g=a\, thus confirming that endogenous oxytocin has a facilitatory role in luteolysis via prostaglandin production.