Plasma progesterone concentrations were measured by radioimmunoassay in the oldfield mouse (Peromyscus polionotus) and the deermouse (P. maniculatus) at estrus and at 5-day intervals during pregnancy. 2. P. polionotus had significantly higher progesterone levels, both at estrus and during gestation. 3. The level was markedly less in female P. polionotus bearing interspecific (P. polionotus x P. maniculatus) hybrid conceptuses than in those with conspecific conceptuses. 4. It is postulated that the reduced progesterone level in females bearing hybrid fetuses may be responsible for reproductive failure which regularly occurs when this cross is attempted. 5. An endocrine difference of this magnitude could represent an incipient reproductive isolating mechanism.
