In 1906, Slator measured the rate of alcoholic fermentation by connecting a Hg manometer to the evacuated culture flask and observing the decrease of vacuum. Rahn''s fermentometer varies from this only in not using vacuum, but measuring pressure increases. While for most comparative experiments, the pressures expressed in mm. of Hg will be sufficient, the absolute amounts of CO2 produced can be easily computed if the volume of the enclosed air space in culture flask and manometer is known. With large amounts of yeast (0.5-1.0 gm. of compressed yeast in 50 cc), the rate of fermentation can be determined in 5 min.