Four factors were tested for their effects on numbers of embryos obtainable from mice injected with pregnant mare serum gonadotropin (PMSG) and human chorionic gonadotropin (hCG) to induce superovulation. The factors tested were: (1) season of year; (2) environmental temperature (in the range of 17.8-23.7 degrees C); (3) age of breeder male mice (in the range of 10-76 weeks); and (4) size of female mice (in the range of 16-30 g body weight). Each of the first three factors was tested for its effect on mating and consequently on embryo yield; the fourth was tested for its effect on numbers of embryos yielded by those females that did mate. In all experiments hybrid female mice (C57BL/6J X CBA/J) were injected with 5 IU of PMSG and 5 IU of hCG and were crossed with males of the CD-1 strain. None of the factors tested was found to have a statistically significant effect on embryo yield. Other factors that should be examined for their effects on mating and/or ovulation are discussed.