1. Phytase (EC 126.96.36.199) concentration has been measured in the small intestine of rat, rabbit, guinea-pig and hamster. Levels varied from 0.12 units (μg phosphorus released/min)/mg protein in the rat to 0.03 units/mg protein in the rabbit. 2. The enzyme is localized in the brush border of the small intestine of the rat. 3. It is suggested that the levels and location of phytase are an important factor in the uptake of metals from metal-phytate complexes. Metal ions released in the immediate vicinity of the absorptive surface of the intestine could be absorbed before being rendered insoluble by competing reactions such as hydrolysis.