An arrest in laying associated with either a polyovarian follicle (POF) or a polycystic ovarian follicle (PCOF) syndrome has been reported in turkey hens photostimulated at an early age with a constant-light photoperiod. Hens expressing the POF or PCOF syndrome had stopped laying for several weeks, but the ovary contained an increased number of mature-size and larger follicles (POF hens), which were cystic (PCOF) in some of the hens. Hens with the POF or PCOF syndrome had plasma progesterone (P₄) concentrations that were relatively high and without surges. We hypothesized that high plasma P₄ concentrations may block ovulatory surges of LH but not the growth or maintenance of hierarchical follicles leading to development of the POF or PCOF syndrome in turkey hens. In the first six studies, hens were photostimulated with either a 14L:10D or a 24L:0D photoperiod and, after laying for 1–38 wk, were then injected daily for up to 14 days with P₄ (up to 1.50 mg kg⁻¹ day⁻¹) and necropsied. At all ages, the oviposition rate was reduced at a P₄ dosage of 0.17 mg kg⁻¹ day⁻¹. With dosages of 0.33 mg kg⁻¹ day⁻¹ or greater, however, ovipositions stopped in most hens within approximately 2 days. For hens laying for less than 15 wk, oviductal weight and number of hierarchical follicles of P₄-injected hens were not different from control vehicle-injected hens, but the numbers of mature, cystic, and atretic follicles were increased. For hens laying for 38 wk, when treated with P₄, oviductal weight and number of hierarchical follicles decreased, but number of atretic follicles increased. No effect of photoperiod was found on egg production, oviductal weight, or follicle number, and none of the hens developed POF or PCOF syndrome in these experiments. Two additional experiments were conducted with hens early in the reproductive period that had been photostimulated with 14L:10D or 24L:0D and injected with P₄ (0.33 mg kg⁻¹ day⁻¹) for 10 or 12 days but not necropsied until 3 wk after the last injection. Most of the hens photostimulated with the 24L:0D photoperiod and injected with P₄, and a few of the hens photostimulated with the 14L:10D photoperiod and injected with P₄, had developed the PCOF syndrome when necropsied. The hens with the PCOF syndrome had high levels of P₄ when necropsied. From these studies, we concluded that the PCOF syndrome can be induced early in the reproduction period by photostimulating turkey hens with a 24L: 0D photoperiod, injecting them for 10 to 12 days with P₄ at a dosage of 0.33 mg kg⁻¹ day⁻¹, and then waiting 3 wk for the PCOF syndrome to develop.