The mechanism(s) of electrolytes (JNa, JC1, JCa and JK) and water transport (JV) were studied in the upper jejunum of the laying hen by an in vivo perfusion procedure. Water secretion is modified by the difference of osmotic pressure between the lumen and plasma and we have estimated the osmotic permeability coefficient (P osm = 17.4 microliter x h-1 x mosm-1 x g-1 DW) and the reflexion coefficient (0.72). At sodium concentrations of 0 and 80 mM x 1-1 in the lumen, there is a large secretion of Na. When the water flow is modified by osmotic pressure, this secretion is markedly influenced by JV. Similarly net chloride flux occurred mainly by solvent drag. Conversely water flow does not modify the large potassium absorption along the concentration gradient and only slightly affects the flux of calcium. Transport of calcium occurred along the gradient of concentration between lumen and plasma without saturation until a lumen concentration of 20 mM x 1-1. This driving force seemed to be the main one since JCa is close to zero when there is no gradient. This observation supports the hypothesis that the variations of net Ca absorption during the laying cycle is due to a modification of concentration of soluble calcium in the contents of the intestine.