The spectrum of action of nitroimidazoles ranges from anaerobic bacteria to aerobic bacteria which are also capable of growth, although sometimes with difficulty, in anaerobic atmosphere and to protozoa without and with mitochondria; in the latter case the anti-protozoal activity depends on the importance of the extramitochondrial enzymes which respond to nitroimidazoles. The more anaerobic the organisms, the greater their sensitivity. In the most sensitive protozoa, the trichomonads, most of the enzymatic reactions inhibited by these drugs take place in the hydrogenosomes; these are cytoplasmatic organelles surrounded by a membrane whose disintegration, in fact, is the first visible effect of the action of the nitroimidazoles. Various nitroimidazoles act with different speed on these and other protozoa, whose membranes can difinitely possess different degrees of permeability towards substances with different chemico-physical characteristics. In the treatment of trichomoniasis, the specific speed of action of a drug is perhaps as important as its half-life in man.