Relationship of conceptus number to the action of luteinizing hormone-antiserum in the pregnant rat.

Treatment of pregnant rats on Day 12 of pregnancy with luteinizing hormone-antiserum (LH-AS) (equine anti-bovine LH) had no effect on the maintenance of pregnancy or serum progesterone levels in rats bearing either 2 or more than 8 conceptuses. In rats with only 1 conceptus, LH-AS administration resulted in a significant decline in serum progesterone to basal levels within 24 h and all animals (n = 6) failed to maintain pregnancy. All rats with either 1, 2 or greater than 8 implantations showed no effect of normal horse serum (NHS) administration. There was no effect of LH-AS administration on serum testosterone levels, compared to NHS-treated controls.