The effects of plant phytase on the gastric degradation of phytic acid and digestibilities of DM and P, and their diurnal variation were evaluated in pigs from 90 to 115 kg BW fitted with simple duodenal T-cannulas. Three diets were fed to three pigs in four collection periods according to a cross-over design. The diets were a wheat-barley-based diet (phytase activity 1005 phytase units [FTU] kg-1), a maize-based diet (91 FTU kg-1) and this maize-based diet mixed with 30% wheat (342 FTU kg-1). Digestibility of DM in the duodenal samples was zero in all treatments. Digestibility of P was high (17%) and equal for all treatments. The diurnal disappearance ratios of DM and P were similar for all diets. Their levels were dependent on gastric retention time. The lowest ratios were found 3 h post-feeding; maximal ratios were observed prior to feeding. The gastric degradability of phytic acid was 3% in the maize-based, 31% in the wheat-maize-based and 47% in the wheat-barley-based diets. The diurnal disappearance ratio of phytic acid degradation was similar for all treatments, and dependent on gastric retention time. Marked differences in degradability levels were observed among the diets.