AbstractTwo correlational studies were undertaken to expolre the relationship between the sex ratio of the pups that a female Mongolian gerbil gestated and her behavior, morphology, and hormone levels late in pregnancy. In the first study, we found that the change in sex ratio between the first and second litters that a female gerbil delivered and the change in her frequency of scent marking late in her first and second pregnancies were significantly correlated. In the second study, we found significant positive correlations between both the percentage and the number of males in the litter a female delivered and (1) her plasma testosterone levels, (2) the size of her ventral gland, and (3) her frequency of scent marking, all measured late in pregnancy. Our data were entirely consistent with the hypothesis that fetal males excrete biologically significant quantities of testosterone into their dam's bloodstream and that this testosterone masculinizes both the behavior and morphology of dams late in pregnancy. © 1993 Johan Wiley&Sons, Inc