Because of the short biological retention half-time of injected Zn-DTPA and the desirability of a maintained blood level of the chelate, alternative modes for its administration were explored. Aqueous Zn-DTPA labeled with 65Zn was dried and ground to a fine powder and incorporated into implants of various combinations of cholesterol, polyethylene glycol 4000, beeswax, and peanut oil in order to produce a slow release of the chelation agent. Although the 65Zn tracer is known to be somewhat labile and exchanges with body stores of zinc and other metals, it was felt at the outset of the experiment that it would be an adequate tracer for Zn-DTPA in this study. Various types of implant pellets placed subcutaneously in the dorsal neck of young adult beagles produced a 24-hr excretion of 65Zn ranging from 3 to 83% of the implanted amount, increasing with the percentage of polyethylene glycol 4000 in the pellet. When 241Am citrate was injected intravenously into beagles pre-implanted with a DTPA pellet, the fraction of the injected 241Am excreted in the first 24 hr increased from 9% (control) to 38% (about 2.9 μmole DTPA/kg/day) to 84% (about 70 μmole/kg/day).