Traditionally, the majority of drug testing on mice has been on male mice. This is due to the hormones of the female mice influencing test results. Sherry Martz, PhD is director of the Organization for the Study of Sex Differences. She explains: "A mouse's menstrual cycle is just four to six days long. . . You have to test each mouse to see where she is." This process is delicately described as "a little like performing a tiny PAP smear."
Source: Now you will have to take this with a bit of faith in me because I could not find this article/information on the Internet. But I happened to stumble across this and could not pass it up. If you pick up "O The Opera Magazine" September 2007, Volume 8, Number 9 and turn to page 214, you will find this information in the article titled: "Of Mice and Women."