Fermentation is a cornerstone phenomenon in Cartesian physiology, accounting for processes such as digestion or blood formation. I argue that the previously unrecognized conceptual tension between the terms ‘fermentation’ and ‘concoction’ reflects Descartes's efforts towards a novel, more thoroughly mechanistic theory of physiology, set up against both Galenism and chymistry. Similarities with chymistry as regards fermentation turn out either epistemologically superficial, or based on shared earlier sources. Descartes tentatively employs ‘fermentation’ as a less teleological alternative to ‘concoction’, later renouncing the (...) explicit use of the term, possibly to avoid chymical overtones. However, his continued use of analogies with fermentative processes in the natural world and in winemaking, coupled with a strong ontological commitment (the stance that the physiological processes are actual fermentations), leads to a reintroduction of natural teleology in his medical system, which I argue may be understood in an Aristotelian sense of ‘simple necessity’. The paper reveals a more nuanced account of Cartesian fermentative medicine, delineating some of its tensions with regard to chymistry as they play out in the dynamics of fermentation and concoction, and linking the analogies to fermentation processes to the difficulties in erasing teleology altogether. (shrink)