Juvenile male Djungarian hamsters (Phodopus sungorus sungorus) exhibit rapid and robust increases in both serum FSH and testicular weight within 1 week of transfer from nonstimulatory short days to stimulatory long days, while serum LH levels rise little or not at all during this same time period. Two experiments were performed to determine whether the rapid increase in serum FSH is responsible for the initial increase in testis weight. In the first study, the rapid increase in serum FSH levels, which normally occurs in response to photostimulation, was suppressed by twice daily injections of porcine follicular fluid, a substance known to contain the FSH-suppressing protein inhibin. Suppression of serum FSH levels during the 7 days of photostimulation resulted in an inhibition of testicular growth. In the second study, young male Djungarian hamsters housed in a short photoperiod were implanted with Alzet minipumps containing ovine (o) FSH in order to experimentally raise serum FSH levels. Another group of animals exposed to short days received minipumps containing oLH. Administration of oFSH, but not oLH, induced testicular growth comparable to that in animals transferred to a stimulatory photoperiod. Furthermore, administration of oFSH was able to overcome the inhibitory effects of porcine follicular fluid on the testes. These results suggest that the rapid increase in serum FSH after photostimulation is the primary signal for initiating testicular development in this seasonally breeding species.