The effect of the pre- and postnatal daylength on the start of spermatogenesis and further testicular development from day 4 up to day 127 was investigated in Djungarian hamsters. Hamsters were either gestated under long (16 h light:8 h dark) photoperiod and reared under long or short (4 h light:20 h dark) photoperiod after birth (L/L and L/S hamsters, respectively), or gestated under short photoperiod and transferred to long photoperiod after birth (S/L hamsters). In L/L and L/S hamsters, spermatogenesis started between day 4 and day 5 (day of birth = day 1), when the first gonocytes entered the S-phase. A, Intermediate and B spermatogonia were first observed on days 6, 8 and 9, respectively. The proliferation pattern of gonocytes and Sertoli cells, studied between day 4 and day 9, did not differ between L/L and L/S hamsters. Hence, the duration of the postnatal photoperiod had no effect on the start of spermatogenesis. The first effect of postnatal photoperiod on spermatogenic development was observed on day 15, when testis weights and tubular diameters were reduced in L/S animals. From day 22 onwards, spermatogenesis was arrested mainly at the mid-pachytene stage, no tubular lumen was formed, and the number of preleptotene spermatocytes was reduced. The ultimate number of Sertoli cells per testis was not affected by postnatal short photoperiod. The duration of the prenatal photoperiod had a clear effect on spermatogenesis after birth. In S/L hamsters, the number of gonocytes per tubular cross-section was reduced on day 4 and 4.5. Gonocyte proliferation was reduced on day 5 and spermatogenesis started one day later. Consequently, A and Intermediate spermatogonia appeared on day 7 and 9, respectively. Sertoli cell proliferation was also shifted to later ages, but the ultimate number of Sertoli cells did not differ from L/L or L/S hamsters. From day 29 onwards, the number of preleptotene spermatocytes was increased in S/L hamsters, indicating that the Sertoli cells in these animals could support more germinal cells. In conclusion, a short postnatal photoperiod does not affect spermatogenesis before day 15 after birth, when further testicular development becomes arrested. A short prenatal photoperiod delays the start of spermatogenesis by one day, alters the proliferation pattern of Sertoli cells, and from day 29 onwards, enables the Sertoli cells to support more germinal cells. The duration of the pre- and postnatal photoperiod did not affect the ultimate number of Sertoli cells.