Italian data furnish a point of departure for an interpretation of conversion (or zero-suffix derivation) phenomena as morphological metaphors, a formalization of which is presented. Previous accounts of lexical class shifting without overt derivational affixation are critiqued, and a syntagmatic derivation process, in which the derivation is marked by the relation between two members of a syntagm, is proposed instead. Conversion is defined as a word formation rule in which the base signifier is unchanged, and an analogy with semantic metaphor is schematized. The typologically isolating character of this phenomenon is established, and implications for the identification of lexical heads are considered. Anna M. Thornton's hypothesis regarding the role of Italian thematic vowels in conversion is critiqued in this framework. (Copyright 1994, Sociological Abstracts, Inc., all rights reserved.) (J. Hitchcock in LLBA 1994, vol. 28, n. 1)