OBJECTIVES: Previous studies in the female sheep have shown that photoperiod modulates the passage of sex steroids between the blood and the cerebrospinal fluid (CSF) and have suggested the involvement of efflux transport. The objective of the present work was to assess the presence of P-glycoprotein (Pgp), which could be responsible for this transport at the level of the choroid plexuses (CP). METHODS: We used flow cytometry and the UIC2 monoclonal antibody against Pgp, to demonstrate the presence of Pgp in the epithelial cell fraction isolated from the sheep choroid plexuses (CPEC). Thanks to the size of the brain structures in sheep, we analyse separately the CP from 4th ventricle (4V) and the CP from the lateral ventricle (LV). RESULTS: In the whole population isolated from the CP, the 4V contained a higher percentage of living, epithelial cells than the LV. The immunoreactive cells to the UIC2 antibody i.e. bearing activated form of Pgp, represented 26.8% of the CPEC in the 4V, and 39.3% in the IV (P<0.05). CONCLUSION: Pgp is expressed in the CPEC from the sheep. Differences in the expression of Pgp between CPEC from IV and 4V might suggest in vivo functional differences between LV and 4V CP in sheep.