Background Oestrogen therapy helps prevent bone loss in postmenopausal women and corrects a decline in Ca absorption efficiency at the onset of menopause. However, the mechanism by which 17 beta-oestradiol (17 beta-E-2) stimulates Ca absorption is unclear. Oestrogen may exert its effect indirectly via increasing 1,25-dihydroxycholeciferol (1,25 (OH)(2)D-3) or its receptor, or act more directly on the intestines via the oestrogen receptor (OR). Since oestrogen also increases retinol levels, this may influence Ca absorption.Aim To investigate the effect of 17 beta-E-2 alone and in combination with 1,25 (OH)(2)D-3 on intestinal Ca uptake and absorption in Caco-2 cells cultured under deplete- and replete-9-cis retinoic acid (9-cis RA) conditions.Methods Twenty-one day-old Caco-2 cell monolayers (n 9 wells per treatment) were exposed to 9-cis RA-deplete and -replete media containing dimethyl sulfoxide (control), 10 nM-1,25 (OH)(2)D-3, 10 nM-17 beta-E-2, or 10 nM-1,25 (OH)(2)D-3 plus 10 nM-17 beta-E-2, for 48 h.Results 1,25 (OH)(2)D-3 stimulated Ca uptake, total Ca transport, calbindin D-9K and CaT1 mRNA levels, while 17 beta-E-2 and 9-cis RA had no effect on Ca absorption or uptake. Nor did they augment the stimulatory effect of 1,25 (OH)(2)D-3.Conclusion These in vitro findings suggest that oestrogen does not have a direct effect on intestinal Ca absorption.