Digestibility of phytate phosphorus (P) will be determined in laying hens on two diets differring in endogenous phytase activity. Layers of different age (21 and 47 weeks) will be used. Calcium and P fractions in excreta will be assayed. Distribution ofphytase activity in the digestive tract of hens, effect of age, diet and pH on the activity will be determined. A hypothesis will be tested that in laying hens fed a diet containing high proportion of wheat, the dietary phytase activity, phytase activityof microorganisms and mucosal phytase activity are high enough to hydrolyze dietary phytate to a significant extent. Wheat-based diets are widespread in the Central Europe, whereas in other countries maize-based diets with low endogenous phytase activityare common.