Sheffield-Wistar albino and PVG hooded females reared under standardized conditions were mated at the ages of 8, 12, 16 and 20 wk during each month of the year. There were 4-8 animals in each group and the time of appearance of vaginal plugs, size of litters and rearing success to weaning at 14 days were recorded. PVG strain performed less well at every stage than the Sheffield-Wistar. Although both strains showed decreased success with greater age at mating, the PVG animals deteriorated more drastically. A seasonal rhythm was noticeable for both strains and was most marked in the weaning success of the older animals. This was most exaggerated in the PVG strain. There are evidently strain differences indicative of a genetic relationship for low fertility and speed of age deterioration in the reproductive system.