Previously we demonstrated gender differences in Na,K-ATPase (NKA) expression and function after renal ischaemia-reperfusion (I-R) injury. Postischaemic membrane destruction causes inhibition of NKA, while heat shock protein (HSP)72 helps to preserve it. We tested the sex differences in postischaemic expression of HSP72 and co-localization with NKA. The left renal pedicle of uninephrectomized female (F) and male (M) Wistar rats was clamped for 55min followed by 2 (T2), 16 (T16) 24 hours (T24) of reperfusion. Uninephrectomized, sham-operated F and M rats served as controls. Postischaemic blood urea nitrogen (BUN), serum creatinine and renal histology were analyzed. HSP72 mRNA expression was detected by RT-PCR, protein levels by Western blot. Fluorescent immunohistochemistry was performed to evaluate the localization of HSP72 and NKA alpha-1 subunit. Postischaemic BUN and creatinine were higher and renal histology showed more rapid progression in M versus F (P<0.05). HSP72 mRNA expression was higher in F versus M in control and in all I-R groups (P<0.05). Similar changes were observed in HSP72 protein levels (F versus M, P<0.05, controls, T2, T16 T24, respectively). Immunohistochemical localization of HSP72 and NKA alpha-1 was similar in control F and M. In postischaemic F kidneys the majority of NKA alpha-1 and HSP72 was co-localized on the basolateral membrane of tubular cells, while in M prominent staining was observed in the cytosol and apical domain. This study indicates that in female kidneys the higher basal and postischaemic levels of HSP72 and different co-localization with NKA might contribute to the gender differences in renal I-R injury.