Expression of proteins in eucaryotic cisterns is often the only way to ensure the correct folding and processing necessary for protein function. Heterologous proteins, however, are commonly retained in the secretory pathway : so that secreted product yield is low despite a high level of transcription. A major limiting step in protein secretion is protein folding in the lumen of the Endoplasmic reticulum. This process is assisted by accessory macromolecules resident in this compartment, including chaperones such as the hsp70 homologue binding protein (BiP). Although induction of foreign proteins in yeast initially elicits a transient increase in local chaperone concentration, long-term protein expression lowers both chaperone and secreted product. A mechanistic model that can account for the experimentally observed role of BiP in secretion and the effects of BiP overexpression on the secretory pathway is described here. The model predicts that equimolar synthesis of chaperone and foreign protein should optimize protein secretion.