Electroencephalogram (EEG) burst-suppression can be produced with several anesthetic agents. Discussions of burst-suppression suggest that it has been viewed by many as a relatively uniform physiologic state independent of the agent used to produce it. This view may be an oversimplification. In this study, relatively deep EEG burst-suppression (suppression to burst time ratio, 4:1) was induced in rats with isoflurane (I), thiopental (T), etomidate (E), and propofol (P). Burst duration, maximum peak-to-peak voltage, area under the curve, and the ratio of power in high versus low frequencies of EEG recorded in both cortex and subcortex (thalamus) were determined. Analysis of the bursts revealed significant differences in duration [I, 1.4 ± 0.4 (SD); T, 0.8 ± 0.5; E, 0.3 ± 0.1; P, 0.4 ± 0.1 seconds], peak-to-peak voltage (I, 488 ± 146; T, 285 ± 106; E, 310 ± 87; P, 249 ± 50 μ V), and area under the curve (I, 111 ± 24; T, 35 ± 31; E, 17 ± 7; P, 21 ± 4 μ V-s) for all agent pairs except etomidate and propofol. Suppression phase analysis revealed considerable residual activity with all four agents, although peak-to-peak voltage (I, 129 ± 29; T, 64 ± 20; E, 62 ± 11; P, 40 ± 15 μ V) and area under the curve (I, 73 ± 17; T, 37 ± 14; E, 30 ± 5; P, 22 ± 10 μ V-s) were greatest with isoflurane. The cortical versus subcortical comparison revealed, for all agents, greater peak-to-peak voltage and area under the curve in the subcortex. The data indicate that the electrophysiologic characteristics of burst-suppression vary among the four agents, with the possible exception of etomidate and propofol. The data suggest that the neurophysiologic states associated with burst-suppression produced by various anesthetics should not be assumed to be uniform.