When rats are supplied with zinc by the diet, absorption interferes with the determination of the efficiency of utilization of zinc in metabolism. In studies on the metabolic efficiency of zinc it was therefore attempted to by-pass the step of absorption by i.m. or i.v. injection and, at the same time, to determine the appropriate dosage for these zinc injections. For that reason, young male rats, after 20 days of zinc depletion were injected i.m. or i.v. in five different dosages of ZnCl2 solutions and their gains in live weight were followed. It was found that higher zinc doses given i.v. were lethal to rats, whereas they were tolerated when given i.m. The better metabolic efficiency of zinc, indicated by comparatively higher live weights of the i.v. injected groups, could be seen even at the lower i.v. zinc dosages. Therefore, based on the results of the present experiment, a dose of 0.4 mg zinc is proposed for the i.v. injection to study the metabolic efficiency of zinc in rats.