Patients with severe constipation are treated with combinations of several different laxatives. The purpose of this study is to examine whether the concomitant use of different laxatives enhances the laxative effect, using an osmotic laxative, magnesium sulphate (MgSO₄), and a stimulant laxative, bisacodyl. The faecal water content of rats, to which MgSO₄ and bisacodyl were coadministered, was lower than that in the MgSO₄ group, while the change in the faecal water content over time was very similar to that in the bisacodyl group. The mRNA expression of the osmotic pressure marker, sodium/myo-inositol transporter, in the coadministration group 5h after the administration was significantly higher than that in the control group and almost equal to that in the MgSO₄ group. The protein expression level of aquaporin-3 (AQP3), which plays an important role in water transfer, in the coadministration group decreased compared to the control group, as was the case in the bisacodyl group. The results of this study indicates that the coadministration of MgSO₄ and bisacodyl does not enhance the laxative effect because the expression level of AQP3 in the colon in the coadministration group was almost equal to that in the bisacodyl group.