Day 11 rat embryos produced by overnight and morning short-period breeding regimens were compared for differences in interlitter variability and stage of development. Female Sprague-Dawley rats were mated either overnight (5 PM-9 AM, n = 22) or for 3 hr in the morning (8-10 AM, n = 20), and the presence of a vaginal plug was determined (day 0). At 11 AM on day 11 of gestation, rats were killed and embryos were examined for crown-rump length, somite number, and general morphology; protein content was determined later. There were no differences in mean number of implantations, live embryos, and abnormalities between the two regimens. There were significant differences in mean crown-rump length, somite number, and protein content but not in the magnitude of their variances. The difference in mean somite number was 1.2 (23.7 vs. 24.9), which corresponds to a time difference in development of approximately 2 hr. The range of mean somite number in morning short-period breeding group was 3.2 (22.2-25.4), corresponding to approximately 5.3 hr. These results are inconsistent with the popularly held hypothesis that the timing of fertilization is the major cause of interlitter variability.