In this paper, I will argue that the connection of morphological patterns to metonymy is fundamental to the efficiency of the lexicon. I will focus on agent noun and denominal verb formation and claim that both constructions involve metonymic patterns. From a cognitive point of view, the lexicon can be defined as a set of symbolic forms we use in speech and writing. This set is expansible, according to our conceptual and communicational needs. Word-formation patterns optimize lexical expansion: they are essential for the efficiency of the lexicon as a symbol storage system which requires immediate lexical access in order to provide immediate communication. Metonymy is crucial to lexical efficiency. Under a conceptual view of metonymy, “a conceptual entity, the vehicle, provides mental access to another conceptual entity, the target, within the same idealized cognitive model.” (Radden & Kövecses 1999:21). According to Langacker, metonymy is so prevalent because it is basically a reference point phenomenon, that is, the entity represented by the metonymic expression “serves as a reference point affording mental access to the desired target.” (2000:199). Metonymy is thus fundamental for an efficient lexicon: as we can mentally access a conceptual entity by means of another, we can neutralize the immediate lexical access and communication problem. We now turn to the relevance of metonymy in agent noun and denominal verb formation. Traditionally, agent noun formation is syntactically described; but the adequacy of syntactic analyses for agent nouns is illusory, as shown by Panther and Thornburg (2003), among others. In professional agent nouns, for instance, we have the cognitive model of the professional market, which establishes a structure of functions and categories. The reference to a worker is not made just by what one does; rather, the ICM of the work market determines which activity can stand for the acts performed in a given profession or agent role; this activity is represented by the verb in a morphological [[X]v Y]n agent noun construction. For instance, 'painter' is connected to two different cognitive models (with different acts, stereotypes, etc.), the one of the art market and the one of house finishing; the social role student is accessed by means of the verb to study, but what characterizes a student is not studying, but being enrolled; etc. So, in agent noun formation a morphological pattern connects to a metonymic model. (Margarida Basilio)