Zinc kinetics were studied and compared after oral simultaneous administration of two tracers, radioactive (Zn-65) and stable (Zn-70) isotope, to four normal human volunteers. Both tracers and zinc concentration were measured in plasma, red blood cells (RBC), urine, and feces for up to 78 days. Radioactive zinc was also measured by external counting over whole body, liver, and thigh. Data from each individual were analyzed using a compartmental model for zinc metabolism. Values calculated for absorption, fractional zinc excretion in urine, exchange with RBC, and secretion into gut using Zn-70 data did not differ from values calculated using Zn-65 data. Results show that human zinc metabolism can be investigated using stable isotopes as tracers to determine parameters of whole body zinc metabolism, including zinc absorption, excretion, and secretion.