In this study we tested the hypothesis that photoperiod can modulate steroid access to the brain in a seasonal breeder. To this goal, we compared the passage of exogenous progesterone to the brain of female sheep maintained under short (SD) or long (LD) daylengths. In the first experiment, we studied two groups of ovariectomized females maintained under SD or LD, for three artificial cycles, consisting of bearing a subcutaneous oestradiol implant (E2-treated) and an intravaginal device releasing progesterone (CIDR). During the third cycle, the concentrations of progesterone and of its metabolites 5alpha-dihydroprogesterone and 3alpha-hydroxy-5alpha-pregnan-20-one were measured in the preoptic area (POA). The levels of progesterone in the POA were higher in ewes under LD than under SD while the amounts of metabolites were unchanged. In the second experiment, we compared ovariectomized female sheep equipped with a cannula in the third ventricle to sample the cerebrospinal fluid (CSF) under LD vs. SD. After progesterone (1 mg and 10 mg) was injected into the carotid artery, it was only detectable in the cerebrospinal fluid in sheep under LD. In the third experiment, we compared progesterone concentration in plasma and CSF in two groups of SD vs. LD ovariectomized E2-treated ewes for 2 h under CIDR treatment. Despite similar progesterone plasma concentrations, concentration in the CSF was 2.5 times higher in SD than in LD. Our results suggest a physiological modulation of the passage of progesterone to the brain according to the photoperiod.