Spermatozoa from C57BL/6NNia mice (7-and 25-month-old males that produced offspring and 25-month-old males incapable of producing offspring which either mated or did not mate after being paired for 1 month with proven-fertile females) were tested in in-vitro fertilization studies. The 7-month-old males fertilized the largest number of oocytes (80\p=n-\86%) in vitro and 79% of them subsequently developed into blastocysts in culture. Aged males which failed to mate fertilized the lowest number of

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... oocytes (11\p=n-\19%) with 48% developing to blastocysts. This group of mice had the lowest number of spermatozoa in the cauda epididymidis (3\m=.\2\ m=+-\ 0\m=.\4\ m=x\105/mg tissue) with fewer motile spermatozoa (22\m=.\3\ m=+-\5\m=.\1%) than younger males. The percentage of spermatozoa retaining their acrosome after 3 h in culture was higher in aged males which had not mated when compared to younger males that had mated. After 4 h in culture, however, the number of spermatozoa that had lost their acrosome was almost identical in the two groups. Superovulated mice which were artificially inseminated with spermatozoa from 25-month-old mice that had not mated did not become pregnant. Testosterone concentrations were lowest in aged mice not mating. These concentrations may explain the poor behavioural response of these males, but whether they account for the inability of spermatozoa to fertilize ova in vitro or in vivo after artificial insemination is not known.