A toxicological study of 4-[(E)-2-(1-methyl-5-nitro-1H-imidazol-2-yl)-ethenyl]-2-pyrimidinamine (azanidazole, Triclose), a new systemic antitrichomonas agent, was run in laboratory animals in order to evaluate its safety. Studies also covered the effects of azanidazole on reproduction in rats treated prior to mating, on fetal development in rabbits and rats, and on fetal survival and growth in rats treated with it during late pregnancy and lactation. The obtained data indicate that azanidazole was well tolerated when administered as a single dose or repeated daily doses for six months. Furthermore, no adverse reproductive effects and no evidence of teratogenic activity were observed in all of the tested animal species. Survival indices were not affected, and body weight of progeny was normal in all studies on reproduction and peri- and post-natal toxicity.