The study was undertaken to investigate the mutagenic activity of hydrazine hydrate a structural isomer of the nucleic acid base thymidine in barley. The treatments were administered alone or in the presence of maleic hydrazide and X-rays. Hydrazine was able to produce chlorophyll mutations in the M1-generation itself if applied alone, in a mixture or as a last treatment in a sequence. In the M2-generation, however, segregation for chlorophyll mutations was observed in all the treatments. The possible mode of action of the mutagen is discussed.