Several experimental models have been investigated to determine their relative value in assessing the efficacy of systemic trichomonacidal agents in laboratory mice. Infections produced by challenge at all of the sites indicated below have proven to be of some use in evaluating potential trichomonacides; (1) Trichomonas vaginalis injected subcutaneously and in the scrotal sac, or either site alone, (2) Trichomonas foetus injected in the peritoneal cavity and in the scrotal sac, or either site alone, and (3) T. vaginalis injected at a subcutaneous site concurrently with T. foetus in the peritoneal cavity and scrotal sac. Infections produced by inoculation of the scrotal sac with either T. foetus or T. vaginalis appeared to be the most reliable for differentiating the relative antitrichomonal potency of test drugs. This conclusion is based on data obtained with five compounds of the 5-nitroimidazole class and one 5-nitrothiazole derivative.