The unidirectional influx of Ca (measured by in vitro methods) was similar along the intestine of the adult rat, from Treitz ligament to ileo-caecal valve. In in vivo ligated loop experiments, the rate of Ca absorption was similar during 30 min irrespective of site, sex and the presence of L-xylose in the Ca solution. On the contrary, these factors modified the quantity of Ca absorbed. After 30 min, absorption stopped in the male ileum whereas it continued (1) in the male jejunum; (2) in the ileum of the female, and (3) in the male ileum in the presence of xylose (absorption being proportional to xylose concentration). Thus, in the adult rat, the duration rather than the rate is the decisive factor in Ca uptake.