When cells of the yeast Saccharomyces cerevisiae are exposed to a decrease in external osmolarity they respond by a stimulation of the production and accumulation of the compatible solute glycerol. As an important part of this response the expression of the gene GPD1 encoding glycerol-phosphate dehydrogenase is induced several fold. We have shown previously that the High Osmolarity Glycerol (HOG) response pathway is required for normal osmostress induction of GPD1 expression. We have investigated the time course of GPD1 induction under different degrees of osmotic stress. While expression of GPD1 was stimulated rapidly under low salt (0.6 molal NaCl) concentrations, higher salt levels (0.9 molal NaCl) caused a progressively longer lag phase in the induction. In a hog1 deletion mutant the response to low salt was also rapid but transient. At higher salt concentrations the delay in the stimulation of GPD1 expression was more pronounced than in the wild type. In addition we observed that the induction of GPD1 is partially restored in a hog1 deletion mutant carrying a mutation that suppresses to some extent the growth defect at high salt concentrations. Recently it has been shown that the yeast PKC-pathway can be stimulated by hypo-osmotic shock. In a mpk1 deletion mutant lacking the terminal kinase of the PKC-pathway, induction of GPD1 expression appeared to be faster than in the wild type. Upon a reduction in external osmolarity the level of GPD1 mRNA rapidly decreased and this effect was entirely independent of the HOG or PKC-pathway. We conclude that the HOG- pathway only partially contributes to the osmostress-induced activation of GPD1 expression and that other pathways modulate the response. A signalling pathway involved in stress controlled gene expression is the RAS-cAMP pathway controlling cAMP-dependent protein kinase (PKA). We found that high PKA activity caused a reduction in the GPD1 mRNA level and low PKA activity had the opposite effect, as was found for other stress-controlled genes. However, in contrast to the situation with other stress-induced genes, strongly altered PKA activity did not affect the induction under salt stress. In addition we did not observe any induction of the GPD1 mRNA level by heat stress, nitrogen starvation, entry into stationary phase or growth on a non-fermentable carbon source although some of these conditions caused a slight stimulation of the activity of the GPD1 gene product. Thus, it appears that expression of GPD1 is not controlled by a general stress response mechanism as proposed for other stress-controlled genes but may be regulated by mechanisms specifically responding to changes in the relative osmolarity between the cytosol and the surrounding environment.