1. As the first part of a study of pesticide toxicity we report the effects of the solvent dimethylsulfoxide (DMSO) on signal transduction in mutants of Saccharomyces cerevisiae. 2. The enzymes of trehalose metabolism, which are activated and deactivated by a "glucose signal" and by heat shock treatment, were chosen as targets for this study. 3. DMSO was shown to be able to permeate glucose and cAMP. The effects of glucose and cAMP were enhanced by pre-incubating the cells in the presence of DMSO. 4. No effects were observed during the heat shock, suggesting that the solvent acts on the cell membrane. 5. The results suggest that DMSO may be used as a vehicle for small molecules which do not easily penetrate yeast cell membranes, thus providing a new tool for biochemical and toxicological studies.