Guanidino compounds in various chick tissues were systematically analyzed by high performance liquid chromatography, and the changes in guanidino compound levels were studied during chick embryogenesis and development. In the whole chick embryo, arginine (Arg), creatinine (CRN), guanidinoacetic acid (GAA), β-guanidinopropionic acid (GPA), homoarginine (HArg), guanidine (G) and methylguanidine (MG) were detected. Arg, CRN and GAA appeared after the sixth day of incubation, and the other compounds appeared after the eighth day of incubation. All of the compounds, except for Arg which temporarily decreased on the eighth day of incubation, increased as embryogenesis progressed. In the yolk, Arg, CRN, GAA, GPA, HArg, G and MG were detected. Most of them dramatically increased at the late stage of embryogenesis. A slight increase in Arg, CRN and G in unfertile eggs was also observed after incubation. In the albumen, Arg, CRN, GAA, G and MG were detected. Arg, CRN, GAA, G and MG increased during embryogenesis. A slight increase in Arg, CRN and G in unfertile eggs was also observed after incubation. In the chick brain, liver, kidney and heart, Arg, CRN, GAA, GPA, HArg and MG were detected. Guanidinosuccinic acid and γ-guanidinobutyric acid, which are commonly detectable in the mammalian liver and brain, were not detectable in the chick organs. All guanidino compounds in the chick organs fluctuated greatly during development. GPA decreased as the chicks developed and was not detected in the adult organs. These observations suggest that birds have a different metabolic system of guanidino compounds from that of mammals, and that this system is activated during embryogenesis and affected by various physiological factors after hatching. It was also suggested that guanidino compounds, even at very low levels, might have some important roles in animal tissues during development.