Pterin, an analog of guanine, is an electron transfer compound in biological systems. Among the analogs, 6-formylpterin (6FP) has been demonstrated to have many marked physiological and pharmacological activities. In vitro, 6FP converts molecular oxygen to reactive oxygen species (ROS) in the presence of NADH or NADPH under light illumination, with the oxidation of NADH is to NAD(+). In the present study, it has been elucidated that some of 6FP derivatives in which the 3-position of 6FP is modified have such unique activities even in the dark where the most of in vivo biological events occur.