This study was conducted to identify the effects of caffeine on the change of blood chemistry components in the serum of the rat(Sprague-Dawley, female). The experimental groups were divided into 7 groups according to the time lapsed after a single oral administration of 100mg/kg caffeine(that is control, 2, 4, 8, 24, 48 and 72 hrs lapsed group) to the rats. The concentrations of glucose, urea nitrogen, uric acid, creatinine, total protein, albumin, A/G ratio, triglyceride, total cholesterol, HDL-cholesterol, free fatty acid and phospholipid as well as the activities of alanine aminotransferase(ALT), aspartate aminotransferase (AST) and alkaline phosphatase(ALP) were measured in the serum of each experimental groups. The results obtained from this study were summarized as follows ; 1. The concentrations of serum glucose were significantly higher($ ho$<0.01) between 4 (143.0 mg/dl) and 8 hrs(138.0mg/dl) in comparison to the control(101.1mg/dl) after a single oral administration of caffeine(100mg/kg). Whereas there were no significant differences in the concentrations of urea nitrogen, uric acid, creatinine, total protein, albumin and albumin/globulin(A/G) ratio in comparison to the control. 2. The concentrations of total cholesterol and HDL-cholesterol in serum were significantly higher ($ ho$<0.01) between 4(77.4mg/dl, total cholesterol) and 8 hrs(64.7mg/dl, HDL-cholesterol) in comparis to the control(62.8, 46.7mg/dl) after a single oral administration of caffeine (100 mg/kg). On the other hand, the concentrations of triglyceride in serum were significantly lower($ ho$<0.01) after 8 hrs(38.8mg/dl) in comparison to the control(66.5mg/dl). 3. The activities of AST in serum was significantly higher($ ho$<0.05) from 2 hrs(149 U/L) to 8 hrs(178 U/L) in comparison to the control(112 U/L) after a single oral administration of caffeine (100mg/kg). The activities of ALT in serum were significantly higher($ ho$<0.01) at 4(45.5 U/L), 24(49.3 U/L), 48(46.8 U/L) and 72 hrs(42.3 U/L) in comparison to the control (39.7 U/L) after a single oral administration of caffeine (100mg/kg) On the other hand, there were no significant differences in the activities of ALP in comparison to the control.