A study was conducted to investigate the serum progesterone (SP₄) profiles and duration of estrous cycles in the farmed Formosan sika deer (FSD; Cervus nippon taiouanus) during the major breeding season. Five parous, open and non-milking hinds were allotted to collect peripheral blood samples twice weekly for P₄ measurement by radioimmunoassay beginning at the initiation of the rutting season indicated by rutting behaviors of the sexually mature stags. The hinds were polyestrous as proved by cyclic changes of SP₄ levels. After the presumptive estrus shown by the lowest concentration of SP₄ (0.20±0.01 ng/ml), this ovarian hormone markedly elevated on day 7 of the cycle (1.67±0.11 ng/ml), reached plateau (3.15±0.16 ng/ml, P<0.01) during days 11 to 18, and then declined to the basal levels in the subsequent estrus. It is concluded that mean duration of the estrous cycle in FSD during the major rutting season is 19.3 days with a range of 17 to 21 days, and that the patterns of circulating progesterone profiles during the estrous cycles of the FSD are similar to those of other deer species so far investigated.