On Day 9 of pregnancy (day of mating = Day 1), the number of corpora lutea in the right ovary was greater than that in the left (mean +/- s.e.m. 9.3 +/- 0.1 and 6.5 +/- 0.3 respectively; N = 70). Although the percentages of ova fertilized on the left and right side were not different (82% and 94%), the percentage wastage was higher on the left side (20%) than the right (14%). A significant difference in sperm numbers in the right (2.8 X 10(6] and the left (0.5 X 10(6] uterine horns were found 1.5 h after mating in 51 females. Morphometric measurements of the lower uterine luminal size showed that the right side (103.9 mm3) was larger than the left side (88.9 mm3; N = 5). It is obvious that there is structural and functional asymmetry in the ovary and uterus in the golden hamster.