Triethylenemelamine (TEM) was administered in the diet to adult male mice at doses of 0.1, 0.3, 1, 10 or 50 mg/kg body weight for 45 days or at doses of 0.1 or 0.3 mg/kg b.w. for 10 days. As a comparison, male mice were treated intraperitoneally with 5 daily doses of 0.25 or 0.5 mg TEM/kg b.w. At the end of the treatment period, males were mated sequentially with 2 untreated virgin females each for 2 or 3 weeks. Near mid-pregnancy the number of implantation sites and fetal deaths were determined. TEM, administered in the diet at 10 or 50 mg/kg b.w. for 45 dyas, was lethal to male mice. Surviving males from the 1 mg/kg level failed to impregnate any females during the two matings. TEM, given in the diet at 0.1 or 0.3 mg/kg for 10 or 45 dyas, decreased fertility and increased dominant lethal mutations in a dose and time dependent manner. These results were comparable to those obtained from males treated i.p. with TEM at 0.25 or 0.5 mg/kg b.w.