Five groups of sexually mature, Dutch-belted male rabbits were studied. Group I consisted of ten controls. Group II (five males) received one injection of 0·2 mg of triethylenemelamine (TEM)/kg body weight. Group III (two males) received twice the amount of TEM given to Group II, and Group IV (five males) received five weekly injections of 0·4 mg TEM/kg body weight. Group V (five males) received one subcutaneous injection of 0·05 m-mole CdCl2/kg body weight. Spermatogonial divisions were inhibited in TEM-treated animals. In Groups II and III, sperm output reached minimal levels 8 to 9 weeks after treatment. Recovery took place within 6 weeks. In Group IV, sperm output was also reduced to minimal levels by the 8th week. Slow recovery was initiated about 15 weeks after the initial TEM injections. Cadmium chloride caused extensive damage and marked atrophy of the testes, so that extra-tubular tissue accounted for about 75% of the testis. Aspermia was approached within 4 weeks. Little recovery followed during 17 weeks following treatment. The percentage of motile spermatozoa and morphologically normal spermatozoa was markedly reduced in Groups IV and V. All treated groups showed marked changes in the frequency of the eight stages of the cycle of the seminiferous epithelium and depression of fertility. Rapid recovery followed in the TEM-treated groups.