The origin of guanine has been unknown, though there are some reports conce
rning its abiotic synthesis. We show here that guanine, as well as uracil a
nd cytosine, are synthesized from a 90%N-2-10%CO-H2O gas mixture via a comp
lex organic product produced with the high-temperature and rapid quenching
technique. This result implies that a large amount of complex organic matte
r including precursors of bioorganic compounds might have been produced on
the primitive earth after cometary impacts.