From the basic fraction of the ethereal extract of Ginseng Radix (Panax ginseng Araliaceae), 5 methoxypyrazine and 8 alkylpyrazine derivatives [3-isopropyl-2-methoxy-5-methylpyrazine, 2-isopropyl-3-methoxypyrazine, 2-sec-butyl-3-methxoypyrazine, 3-sec-butyl-2-methoxy-5-methylpyrazine, 2-isobutyl-3-methoxypyrazine, 2-ethyl-6-methylpyrazine, 2-ethyl-5-methyl-pyrazine, trimethylpyrazine, 2,6-diethylpyrazine, 2,3-dimethyl-5-ethylpyrazine, tetramethylpyrazine, 2,6-diethyl-3-methylpyrazine and 2-ethyl-3,5,6-trimethylpyrazine] were identified by gas chromatography and gas chromatography mass spectrometry. The methoxypyrazine derivatives were likely to be of major significance in the characteristic earthy, green aroma with moldy undertone of Ginseng Radix. One of them was a novel methoxypyrazine derivative, 3-sec-butyl-2-methoxy-5-methylpyrazine (6), and the structures of 3-isopropyl-2-methoxy-5-methylpyrazine and (6) were estimated from mass spectra of known methoxypyrazine derivatives, 2-isopropyl-3-methoxypyrazine and 2-sec-butyl-3-methoxypyrazine, and confirmed by their syntheses. Compound (76) possessed a characteristic floral, moldy, green aroma with an odor threshold of 2.0 parts per 109 parts of water. The mass spectra of methoxypyrazine derivatives showed similar systematic fragmentations starting from McLafferty rearrangement and .gamma.-cleavage, and the mass spectrometry was a useful method for structural elucidation and detection of methoxypyrazine derivatives.