1. The effect of adverse osmotic pressure gradients on fluid transfer and electrical potential across the wall of sacs of rat everted small intestine was investigated.2. Addition of mannitol to the mucosal fluid produced a potential change of 0.062 mV/m-osM and a decrease in fluid transfer of 0.015 ml./m-osM/hr. This is consistent with the production of streaming potentials due to fluid movement through negatively charged pores in the intestine.3. The solute-linked fluid movement does not pass through these negatively charged pores which are responsible for the streaming potentials.4. From the magnitude and polarity of the streaming potential a value of -50 mV has been calculated for the zeta potential at the phase boundary in the pores.5. Streaming potentials have been used to measure the equivalent pore radius, and a value of 4A has been obtained.6. It is concluded that electro-osmosis is not responsible for fluid transfer by the intestine, and the potential difference associated with hexose transfer is not electrokinetic in origin.