Addition of glucose or related fermentable sugars to derepressed cells of the yeast Saccharomyces cerevisiae triggers a RAS-protein-mediated cAMP signal, which induces a protein phosphorylation cascade. Yeast strains without a functional CDC25 gene were deficient in basal cAMP synthesis and in the glucose-induced cAMP signal. Addition of dinitrophenol, which in wild-type strains strongly stimulates in vivo cAMP synthesis by lowering intracellular pH, did not enhance the cAMP level. cdc25 disruption mutants, in which the basal cAMP level was restored by the RAS2val19 oncogene or by disruption of the gene (PDE2) coding for the high-affinity phosphodiesterase, were still deficient in the glucose- and acidification-induced cAMP responses. These results indicate that the CDC25 gene product is required not only for basal cAMP synthesis in yeast but also for specific activation of cAMP synthesis by the signal transmission pathway leading from glucose to adenyl cyclase. They also show that intracellular acidification stimulates the pathway at or upstream of the CDC25 protein. When shifted to the restrictive temperature, cells with the temperature sensitive cdc25-5 mutation lost their cAMP content within a few minutes. After prolonged incubation at the restrictive temperature, cells with this mutation, and also those with the temperature sensitive cdc25-1 mutation, arrested at the 'start' point (in G1) of the cell cycle, and subsequently accumulated in the resting state G0. In contrast with cdc25-5 cells, however, the cAMP level did not decrease and normal glucose- and acidification-induced cAMP responses were observed when cdc25-1 cells were shifted to the restrictive temperature. These results show that, in the original genetic background at least, growth arrest of cdc25-1-bearing cells at the restrictive temperature is not due to cAMP deficiency. Previous experiments have provided evidence for the presence of a glucose-repressible protein in the signalling pathway. Exponential-phase glucose-grown cells of a strain with overexpression of CDC25 unexpectedly showed a glucose-induced cAMP signal. Control experiments, however, indicated that overexpression of CDC25 caused a defect in glucose repression. Introduction of the cat1 derepression mutation in the strain with overexpression of CDC25 restored glucose repression and abolished the glucose-induced cAMP signal, indicating that the CDC25 protein is not the glucose-repressible component of the signalling pathway.