Summary. A series of experiments was carried out to determine the effect of five metals on the histology and functioning of the testis and epididymis in rats. The metals (silver, copper, tin, nickel and cobalt) were administered subcutaneously as aqueous salts in a single dose or as daily injections from 1 to 30 days; the testes were then subjected to normal histological examination and dark ground microscopy, and to radiography following intra-arterial injection of Micropaque in one experiment. All five metals produced acute and chronic changes in the histology of the testis and interfered, to some degree, with spermatogenesis. All tissues showed improvement following the initial injection, even in spite of continued daily injections. The individual results are described in detail. The effects and possible modes of action of these metals are discussed and contrasted with cadmium. It would appear that all the metals investigated act in a different way from cadmium, and may in fact limit their own action by precipitating proteins in the membrana propria, thus making it impermeable to further metallic ions. It is possible that the action of the metals on the epididymis varies from that on the testis, and the damage produced in the epididymis and ductuli efferentes shows less tendency to recover.