The dose-ralated dffects of intravenous infusion of thiopental and ketamine on the rat EEG were evaluated quantitatively by spectrum analysis of EEG recorded from the rat scalp. The anesthetics were infused into jugular vein at various rates ranging from 0 to 8ug/min/g body weight, and then bipolar EEG was recorded from the rat scalp and tis spectrum were calculated by powere wpectrum analysis. the density of each bands(delta 1-3.25, theta 3.5-7.75, alpha 8-12.15, beta 1.13-17.75, beta 2.18-20.75, and beta 3.21-31.75Hz) and total density were derived from the spectrums. In visual inspection of conventional EEG, low doses of thiopental increased the amplitudes of spinles. but higher doses decreased the amplitube gradually to electrical silence with increase of infusion rates. During infusion of higher doses of ketamine, two types of EEG were identified by the spectral patterns:The one was the cases in which increases of the power density over all frequency ranges were observed, and the other was those in which marked increases of density in specific frequency were observed. In thiopental infusion, the densities of all bands were increased to peak at 1~2ug/min/g and therafter were decreased with higher rates of infusion. In ketamine infusion, the densities were increased when the infusion rate was increased. These results suggest that, by the changing patterns of the band densities dervied from spectrum analysis of EEG, not only the effect on EEG of thiopental of tetamine can be quantified but also their differences of mechanisms of action on brain be reflected.