The research was carried out to study the serum concentrations of blood components in the hypothyroidism, using the Wister strain rats. As a goitrogen, methylthiouracil (methiocil; 6-methy I -2-thiouracil) was used. The compound was suspended in 2% of aqueous sol of carboxymethyl cellulose and the suspension was injected into the stomach or injected subcutaneously. In one experiment, the goitrogen disolved partially in aqueous solution of sodium hydroxide was injected subcutaneously to the rats. The period of administration was 12 hours (4 times of injection, 3 hours interval) or of approximately one week, twice injection a day. The blood was taken at 3 hours following the final administration, and serum of the blood was determined for total protein(TP), albumin(Alb), globulin(Gl), glucose(Glu), blood ureal nitrogen(BUN), cholesterol(Chol), Calcium(Ca), magnesium(Mg), and phosphorus(P). By the administration of the goitrogen, serum concentrations of BUN, Chol, Mg increased and that of P decreased. Ratio of Alb/Gl in the serum concentration also was high in the group treated with methiocil in comparison with the control group. Oral treatment of methiocil increased only glycine among amino acids in the serum. The changes of concentration of blood serum component following the administration of goitrogen indicates that steer producing the beef with high degree of fat deposit is in a state of hypothyroidism.