It remains unclear how anesthetics produce immobility, an end-point used in determining anesthetic potency. Understanding how movement (in response to noxious stimulation) is ablated by anesthetics could be aided by using spectral analysis of the high and low frequency components of complex movement patterns. We therefore applied a spectral analysis to previously published movement data from rats anesthetized with isoflurane and halothane at 0.6, 0.9, and 1.1 minimum alveolar concentration (MAC). We recorded isometric forces of hindlimb movement elicited by noxious mechanical stimulation of the hindpaw. The movement patterns were subjected to spectral analysis to determine force amplitude for each frequency component. When halothane was increased from 0.6 to 0.9 MAC, force amplitude decreased only for the lowest-frequency (<1 Hz) components, in part related to the generally lower high-frequency forces at 0.6 MAC. Between 0.6 and 0.9 MAC isoflurane amplitude was reduced for most frequencies in the 0-10 Hz range. For both halothane and isoflurane at 1.1 MAC, as expected, force amplitude substantially decreased at all frequencies. We conclude that spectral analysis is useful to describe and quantify the effects of anesthetics on complex movement patterns resulting from noxious stimuli applied during anesthesia. CONCLUSIONS Complex movement can occur when a noxious stimulus is applied to an anesthetized animal. The frequency components of these movement patterns can be described and quantified by spectral analysis, thus providing a useful tool to investigate the immobilizing properties of anesthetics.