Aminoguanidine sulfate (AGS) produces a peculiar anomaly in the liver of chick embryos. Incorporation of amino acids and bases into the liver and body was investigated to clarify the mechanism of this effect. After chick embryos of White Leghorns were injected with AGS on the 5th day of incubation, ³H-thymidine (³H-T), ³H-uridine (³H-U), ¹⁴C-glycine (¹⁴C-G) and ¹⁴C-leucine (¹⁴C-L) were each injected once into the albumen at a specified time from the 5th day to the 8th day of incubation. Incorporation of ³H-T into the liver was markedly reduced in the nuclear (or DNA) fraction, apparently being inhibited at 3 to 6 hr, while incorporation into the body was only slightly affected. Incorporation of ³H-U into RNA was only slightly affected in the liver 24 hr after injection, while incorporation into the body did not differ from control. Incorporation of ¹⁴C-G was remarkably reduced with AGS both in the liver and body protein, and the degree of inhibition was around 26-52% at 3 or 24 hr. Incorporation of ¹⁴C-L, however, was reduced only in the liver, and became pronounced on the 8th or the 9th day of incubation. The primary action of AGS seems to be on nucleoprotein synthesis ; namely, on inhibition of DNA synthesis. Those findings are discussed, through comparison of histopathological observations.