The inoculation of active dry wine yeast (ADWY) is one of the most common practices in winemaking. We have used DNA microarray technology to examine the genetic expression patterns of a commercial ADWY strain after rehydration. After rehydration of ADWY for 30 min, a further hour in water after rehydration did not lead to any relevant changes in global gene expression. Expression changes in rehydrated cells upon incubation in a sorbitol solution at the same osmotic pressure as in complete must were rather limited, whereas the presence of fermentable carbon sources or the complete medium (synthetic must) produced very similar transcriptional responses. The main responses were the activation of some genes of the fermentation pathway and of the nonoxidative branch of the pentose phosphate pathway, and the induction of a huge cluster of genes related to ribosomal biogenesis and protein synthesis. The presence of cycloheximide in fermentable medium produced a similar but stronger transcriptional response. Whereas the viabilities of rehydrated cells incubated for 1 h in these different media were similar, yeast vitality, which represents the fermentative capacity of the yeast, showed a positive correlation with the availability of a fermentable carbon source.