Absorption and endogenous excretion of P by male broiler chicks (14-29 d old) were quantitatively evaluated at different Ca:P ratios (1, 1:1; 2, 1.5:1; 3, 2:1; 4, 2.5:1) in four groups given experimental diets ad lib. The P content was the same in all diets. An isotope-dilution technique was used to determine endogenous faecal and renal excretion. Ca and P retentions in the whole body were estimated according to the comparative slaughter technique. P absorption was calculated from retention and endogenous excretion. Absorption and endogenous excretion of P amounted to (mg P/d per chick): 304, 270, 160 and 158; and 135, 109, 31 and 30 in groups 1, 2, 3 and 4 respectively. Widening of the Ca:P ratio in the feed limited the P absorption. Availability of feed P amounted to (%): (1) 66, (2) 57, (3) 32 and (4) 30, and the amounts of absorbed P retained were (%): (1) 56, (2) 60, (3 and 4) 81. The increasing Ca concentration in the feed showed a greater effect on P absorption than on P retention. The ratios of relative retention to relative endogenous excretion of absorbed P were: (1) 1.27, (2) 1.50, (3 and 4) 4.26.