1. 2,4-Dinitrophenol (2,4-DNP) in substrate level concentrations (200 microM-1 mM) temporarily inhibits H2 production by Tritrichomonas foetus and Trichomonas vaginalis as well as the accumulation of metronidazole, dependent on its reduction by the two trichomonad species and by Entamoeba invadens. 2. 2,4-DNP competes for the reducing equivalents which are necessary for H2 production or for the reduction of metronidazole, thereby inhibiting these processes. 2,4-DNP is reduced to 2-amino, 4-nitrophenol. 3. 2,4-DNP in concentrations up to 800 microM has no effect on the uptake of O2 by these organisms. 4. 2,4-DNP has some toxicity for T. foetus.