The single-pass perfusion technique was employed in order to investigate the effect of water flux from the rat jejunum in the normal experimental conditions. Our results suggested that water flux below ${pm}0.75%/cm$ of jejunal length was considered normal. Water flux was $-0.131;{pm};0.311%/cm$ of jejunal length in a citrate buffer and should be corrected in order to determine the permeabilities of the compounds. Perfusion rate up to 0.5 ml/min had no effects on the permeability of ampicillin. Neither the effective permeabilities nor the wall permeabilities of aminopenicillins were influenced by water flux during experiments in rats.