Summary. The size of litters sired by male mice throughout their entire life has been followed. Each male was paired with a new young female approximately every 5 months so as to avoid any effect of ageing of the dam. There were three groups of males (1) entire, (2) unilaterally castrated and (3) 9 months younger than the first two groups. The results indicated that the individual male, regardless of treatment, had a significant influence on the size of litter sired. Unilateral castration, however, did not significantly affect litter size, nor did the age of the male.