Studies on the control of the corpus luteum in the vole, Microtus agrestis

The lifespan of corpora lutea resulting from hormonally induced ovulations was prolonged by exogenous prolactin, concurrent lactation, or pregnancy. Treatment of mated females with bromocriptine resulted in failure of luteal function only when the drug was given before Day 6 of pregnancy. Pregnancy was dependent on the presence of the ovaries in its later stages. The results suggest that prolactin is luteotrophic in early pregnancy but that a placental luteotrophin may become effective by Day 6 of pregnancy.