- The fecal monitoring technique for measuring the absorption of Zn, Mn, Se, and Fe was studied in eight male piglets (mean +/- SEM birthweight (bw) = 1695 +/- 50 g) using high resolution gamma spectrometry. Four d old piglets were fed a complete liquid milk diet for five d prior to the orogastric administration of an isotope dose (75Se, 54Mn, 59Fe, and 65Zn) equilibrated with the liquid milk diet. 51CrCl3 was used as a fecal marker but was found to be partially absorbed. Stool samples were collected daily for 15 d, counted, and then the daily fecal excretion was calculated. Results indicate that endogenous excretion for each of the isotopes was not constant but decreased exponentially with time. The pattern of endogenous excretion varied between elements. An improved method for calculating the endogenous excretion was therefore developed. This method is based on the pattern of endogenous excretion in three-four d old male piglets (mean +/- SEM bw = 2060 +/- 75 g) injected intravenously with the same isotopes and on the level of endogenous excretion in orally fed animals in the postabsorptive phase of excretion. These findings have important implications for the estimation of endogenous excretion in future fecal monitoring absorption studies in order to minimize underestimation of true absorption.