A blue fluorescent substance was extracted from the aqueous fraction of ginseng extract. The effects of this blue fluorescent substance (F4) on the activities of adenylate cyclase, guanylate cyclase, and lactate dehydrogenase were studied. The fluorescent substance enhances the activity of adenylate cyclase from the rat brain in the very low concentration range (50 pg to 50 ng), while the same substance represses the activity of adenylate cyclase in the concentration range greater than 0.5㎍. On the other hand, it was observed that this substance did not affect the activity of guanylate cyclase from the rat brain. Also, the activity of the lactate dehydrogenase from beef heart was not affected at all by the blue fluorescent substance. Although the chemical nature of the fluorescent substance was not fully characterized, the UV spectra of this substance suggest that it is different from the blue fluorescent adenosine isolated from red ginseng by other workers.