A radioisotope dilution technique was adapted for determination of the absorbability of dietary zinc. In 16 growing rats given a casein diet with 12.8 mg Zn/kg dry matter, a mean apparent Zn digestibility of 72.4% was observed in a 6-day balance period 9 days after an intramuscular injection of 65Zn-ZnCl2. There was no significant difference in the specific radioactivity of zinc in blood plasma on day 15 and in urine collected from day 9 to day 15 used to assess the fecal excretion of endogenous zinc. Consequently, there was good agreement between the corresponding mean zinc absorbability (94.8 vs. 95.3%) calculated from these data.