Abstract
Attempts were made to examine progesterone levels during pregnancy block, first initiated by pheromones and then by the injection of pregnant mare serum gonadotropin (PMSG) . The proximity of a strange male inhibits the usual increase in progesterone levels, resulting in no activation of the corpora lutea, from the estrous cycle to the pregnancy. Strain differences were observed in the imitation of pregnancy block by the injection of PMSG. The C 3 H and BALB/c strains required much smaller doses of PMSG for pregnancy block than the C 57 BL /6 strain did. The changes of progesterone observed in pregnancy block initiated by PMSG were slightly different from that caused by the proximity of the strange male : The progesterone levels temporally increased on the day following the injection of PMSG and then decreased on day 4 in the PMSG-treated group. These results suggest that PMSG imitates the effect of the proximate strange male in the pregnancy block.
