Eleven groups of fifty rats each were kept under uniform conditions of management and their life-spans under different treatments were compared. Females lived longer than males, while ovariectomy tended to shorten the life-span of females and castration to lengthen that of males. Implantation of gonadectomized rats with oestradiol benzoate did not prolong the life-span of the females but it did tend to prolong that of males. Implantation of gonadectomized rats with testosterone propionate tended to shorten the average life-span of both sexes. Unbred females outlived bred ones, on the average, but the difference was not significant at the 5% level. Late initial breeding appeared to be more harmful than breeding for the first time at the usual age for laboratory rats. Light breeding had little or no effect upon the average longevity of males. These conclusions are tentative. The results are consistent but they mostly lack acceptable statistical significance. The most significant result statistically was that rats exposed to their own or implanted oestrogens had a longer average life-span than did those exposed to their own or to implanted testosterone.