Germinated seeds of barley, after anaerobic treatment, produce considerable amounts of hydrogen. The anaerobic stress induces hydrogenase activity, as measured by the ability to evolve hydrogen from reduced methyl viologen. Several pieces of evidence, including bacteriological controls and induction in axenic calluses, indicate that the hydrogenase activity is induced by the plant itself and not by contaminant microorganisms. The hydrogenase is selectively induced in roots and to a lower extent in hypocotyls, but no hydrogenase activity can be detected in leaves.