Although eukaryotic G-protein coupled receptor (GPCR) systems are well known for their ability to detect and mediate rapid responses to extracellular signals, the full range of stimuli to which they respond may not yet have been identified. Activation of GPCRs by hormones, pheromones, odorants, neurotransmitters, light and different taste compounds is well established. However, the recent discovery of a glucose-sensing GPCR system in Saccharomyces cerevisiae has unexpectedly added common nutrients to this list of stimuli. This GPCR system mediates glucose activation of adenylate cyclase during the switch from respirative/gluconeogenic metabolism to fermentation. The GPCR system involved in pheromone signalling in S. cerevisiae has already served as an important model and tool for the study of GPCR systems in higher eukaryotic cell types. Here, we highlight the similarities and differences between these two signalling systems. We also indicate how the new glucose-sensing system can serve as a model for GPCR function and as a tool with which to screen for heterologous components of signalling pathways as well as for novel ligands in high-throughput assays.