The effect on the testis of some forty-two water-soluble salts of metals and rare earths was investigated in rats and mice. In general, a single intratesticular injection caused varying degrees of degeneration of the seminiferous epithelium and the interstitium. Thirty-five of the salts tested exerted some degree of antitesticular effect. A single subcutaneous injection was ineffective but continuous administration by the same route caused selective spermatogenic arrest with nine salts. Some of the salts caused aspermia of the recipient animals; others disintegrated the residual spermatozoa in the ductus deferens by separation of the head and tail. The possible mechanism of action of the salts is discussed.