In the yeast Saccharomyces cerevisiae the accumulation of cAMP is controlled by an elaborate pathway. Only two triggers of the Ras adenylate cyclase pathway are known. Intracellular acidification induces a Ras-mediated long-lasting cAMP increase. Addition of glucose to cells grown on a non-fermentable carbon source or to stationary-phase cells triggers a transient burst in the intracellular cAMP level. This glucose-induced cAMP signal is dependent on the G alpha-protein Gpa2. We show that the G-protein coupled receptor (GPCR) Gpr1 interacts with Gpa2 and is required for stimulation of cAMP synthesis by glucose. Gpr1 displays sequence homology to GPCRs of higher organisms. The absence of Gpr1 is rescued by the constitutively activated Gpa2Val-132 allele. In addition, we isolated a mutant allele of GPR1, named fil2, in a screen for mutants deficient in glucose-induced loss of heat resistance, which is consistent with its lack of glucose-induced cAMP activation. Apparently, Gpr1 together with Gpa2 constitute a glucose-sensing system for activation of the cAMP pathway. Deletion of Gpr1 and/or Gpa2 affected cAPK-controlled features (levels of trehalose, glycogen, heat resistance, expression of STRE-controlled genes and ribosomal protein genes) specifically during the transition to growth on glucose. Hence, an alternative glucose-sensing system must signal glucose availability for the Sch9-dependent pathway during growth on glucose. This appears to be the first example of a GPCR system activated by a nutrient in eukaryotic cells. Hence, a subfamily of GPCRs might be involved in nutrient sensing.