This paper presents a study of the breeding history and growth of the golden hamster, from observations made on a colony of 1500 animals over a 2-yr. period. Hamsters should be housed in metal cages and provided with a varied diet of grain and fresh vegetables. Activity of a 4-day cyclic nature has been observed in the [female][female]. This may correspond to the estrus cycle. Matings occur usually at night. No seasonal distribution of litters is apparent. Principal factors affecting breeding are (a) diet, (b) temp., and (c) age of [female][female]. Length of gestation period (15 days 21 hrs.), detd. from time of first copulation to delivery, agrees with that found by earlier workers. Postnatal development is rapid. 8-day-old hamsters can get along without maternal care, in spite of the fact that their eyes do not open until the 14th-15th day of life. In the x, the vagina opens when the animal attains a wt. of 8 gms., and may occur any time between the 8th and 14th day of age. Delivery of litters may occur at an age of 59 days, but mean age at first littering is 73.6 [plus or minus] 9.45 days. Males may copulate by day 30, but fertile matings have not occurred until day 43. The mean litter size, as detd. from 107 cases, is 6.93 [plus or minus] 0.23.