A method for measuring the steady state concentration of hydrogen gas dissolved in rumen contents was developed, which consisted of equilibrating the intercellular fluid with sterile salt solution within a dialysis sac immersed in the rumen or in the in vitro system under study. After about 1 hr of equilibration the contents of the sac were withdrawn and transferred without loss of hydrogen to a flask devoid of materials except water and water vapor. Carbon dioxide was absorbed with alkali and the remaining gas displaced by boiling, with condensation of vapor in a water-cooled hypodermic syringe. The hydrogen was collected in the syringe and analyzed with a gas chromatograph. The rate of methane production correlated more closely with concentration of dissolved hydrogen than with formate. At saturating formate concentrations, addition of hydrogen increased methanogenesis. It is concluded that hydrogen is a very important intermediate in the rumen production of methane.