The effects of three seafoods, shrimp, squid and octopus, on lipid metabolism were investigated in mice fed on 0.1% and 1.0% cholesterol-supplemented diets in the first experiment. One of each of these seafoods and casein were added to the basal diet at levels of 15% and 5%, respectively, as proteins. Casein served as the sole protein source of the control diet. The serum cholesterol concentration was significantly lower in the mice fed on shrimp and squid in the 0.1% cholesterol diet and on any seafood in the 1.0% cholesterol diet when compared with that in the mice fed on the control diet. The liver cholesterol concentration was significantly lower in all seafood groups given the 0.1% cholesterol diet, and in the squid and octopus groups given the 1.0% cholesterol diet. In the second experiment, the effect of these seafoods on lipid metabolism was compared with that of their defatted products in mice fed on a 0.2% cholesterol diet. Defatting resulted in an increase in the serum cholesterol and triglyceride levels in the shrimp and squid groups. The hepatic cholesterol concentration in all the seafood groups was significantly lower than that in the control group, and defatting did not influence the liver cholesterol concentration. Fecal total steroid excretion was higher in all the seafood groups when compared with that in the control group, and was not modified by the removal of fats. Thus, shrimp, squid and octopus exerted hypolipidemic activity; the serum cholesterol-lowering activity of shrimp and squid was attributed to their lipid fraction, whereas the non-lipid fraction of shrimp, squid and octopus contributed to a reduction of hepatic cholesterol and an increase of fecal steroid excretion.