The predicted effect of graded concentrations of phytate P (x) and log doses of phytase (v) on phytate (IP6) and phytate ester (IP5, IP4, and IP3) concentration and in the gizzard of 21-day-old broilers. A. Gizzard IP6 concentration was predicted with the equation: y = 8,829 − 24,618x + 38,375x2 − 5,888v + 1,030v2 + 18,225xv − 28,144x2v − 3,389xv2 + 5,162x2v2, adjusted R2=0.99, RMSE=30.9, P < 0.0001. In the absence of phytase, the predicted concentration of IP6 in the gizzard was 4,524, 4,164, or 5,618 nmol/g in birds fed 0.24, 0.345, or 0.45% phytate P, respectively. The predicted minimum IP6 concentration in the gizzard was achieved with 1,132, 1,285, or 1,208 phytase units (FTU)/kg of phytase at 0.24, 0.345 or 0.45% dietary phytate P, respectively. B. Gizzard IP5 concentration was predicted with the equation: y = 566 − 281v + 46.6v2, adjusted R2=1.00, RMSE=1.77, P < 0.0001. The predicted minimum concentration of IP5 in the gizzard was achieved with 1,028 FTU/kg, regardless of the phytate P concentration of the diet. C. Gizzard IP4 concentration was predicted with the equation: y = 521 − 251v + 41.7v2, adjusted R2=1.00, RMSE=1.60, P < 0.0001. The predicted minimum IP4 concentration in the gizzard was achieved with 1,028 FTU/kg, regardless of the phytate P concentration in the diet. D. Gizzard IP3 concentration predicted with the equation: y = − 914 + 15421v − 24,186v2 + 1186x − 284v2 − 13409xv + 20,950x2v + 2746xv2 − 4240x2v2, adjusted R2=0.95, RMSE=24.7, P < 0.0001. In the absence of phytase, the predicted concentration of IP3 in the gizzard was 491, 519, or 486 nmol/g in birds fed 0.24, 0.345, or 0.45% phytate P, respectively. The predicted minimum IP3 concentration in the gizzard was achieved with 1,462, 966 or 1,828 FTU/kg at 0.24, 0.345 or 0.45% dietary phytate P, respectively. e. The concentration of the sum of all IPs in the gizzard was predicted with the equation: y = 9,371 + 414x + 1,658x2 − 5,709v + 955v2 − 436xv, adjusted R2=1.00, RMSE=47.1, P < 0.0001. In the absence of phytase, the predicted sum of IPs in the gizzard was 6,924, 7,771 or 9,238 nmol/g in birds fed 0.24, 0.345 or 0.45% phytate P, respectively. The predicted minimum concentration of the sum of IPs in the gizzard was achieved with 1,096, 1,170 or 1,245 FTU/kg at 0.24, 0.345 or 0.45% dietary phytate P, respectively.