Understanding music in the Western idiom includes perceptual sensitivity to the tone relationships and hierarchy of stabilities contained in the tonal-harmonic system. These features of the tonal-harmonic system are correlated with statistical properties of tonal music. For example, the frequency with which each tone is sounded has been shown to be closely related to its relative prominence in the tonal hierarchy. Listeners may be sensitive to such regularities, and, it has been argued, may use this information to develop, or elaborate, a sense of tonality for the music. This experimental work has pursued the issue of sensitivity to regularities in music by examining listener’s sensitivity to duration and frequency of occurrence in tone sequences for which construction rules deviated from the rules of Western tonality. Sensitivity will be described both for abstract sequences of pure tones and for musical materials provided by contemporary composers. It is contrasted with an alternative perceptual strategy—that of assimilation to the tonal hierarchy. [Work supported by NSERC.]