Mirskaia & Crew (1930) have reported that the first vaginal oestrus in young female mice was frequently not associated with mating. In addition, pregnancy followed first mating in only 24% of cases whereas in the same mice at 3 to 6 months of age, 80 to 90 % of females became pregnant following mating. Since it seemed not unlikely that genetic and/or environmental factors may have contributed to this poor reproductive performance, it was decided to reexamine the fertility of young mice.
Mice of the Quackenbush (QS) strain were used in this study. They were housed under light- and temperature-controlled conditions and given free access to a commercial pelleted mouse food and tap water. The 'young' female mice were 21 days old at the start of the experiment while the 'mature' females were virgin mice approximately