The study aimed at obtaining basic information about estrous cycle in rats treated with dexamethasone, for polycystic ovaries, induced by constant illumination. It was used 30 female rats (Rattus norvegicus albinus) from the lineage Wistar, with 90 days years old, divided according the following groups: Group I - rats maintained in a light/dark cycle for 12/12 hours, and after 100 days submitted to the cyclicity evaluation (control); Group II - rats maintained under constant illumination during 100 days and after submitted to the cyclicity evaluation; Group III - rats maintained under constant illumination during 100 days and after treated with dexamethasone for five days, and, then, submitted to the cyclicity evaluation. The results showed that after 100 days of experiment, the animals from group I presented a normal cyclicity, being observed the four phases of the cycle. In the animals of groups II and III, it was observed a higher incidence in the estrous phase, with 85% and 76,5% respectively, characterizing the state of permanent estrous. It was observed the phase of diestrous with 15% in group II, and 23,5% in group III, not being observed the phases of proestrous and metaestrous. After treatment with dexamethasone, it was verified a great reduction in the estrous phases in the animals from group III, what was also observed in the animals from group II, reaching numbers of 34,5% and 20,85%, respectively. Yet, there was an increase in the diestrous phase in group II (64,57%), and group III (75%). It has been noticed in these groups the presence of the proestrous phase with 0,92% in group II and 4,15% in group III, not being observed the metaestrous phase. The treatment with dexamethasone during five days produces, more rapidly, a possible retake of the estrous cycle in rats with ovarian polycystic ovaries