The response of derepressed cells of Schizosaccharomyces pombe to the addition of glucose included a marked and reversible activation of neutral trehalase that was not produced in repressed cells. The protein synthesis inhibitor cycloheximide, the protonophore 2,4-dinitrophenol or the uncoupler sodium azide also enhanced trehalase activity in derepressed cells provided glucose was present in the incubation assays. However, only 2,4-dinitrophenol or cycloheximide was able to induce trehalase activation in repressed cells. Stimulation of trehalase by these compounds was preceded in all cases by a rapid increase in adenosine 3'-5'-cyclic monophosphate (cAMP) content. Since exogenous cAMP can activate trehalase both in repressed and derepressed growing cells, the results provide evidence for the existence of an induced cAMP signalling pathway in the fission yeast with several entries for trehalase activation. The correlation between cAMP increase and trehalase activation was not maintained when the enzyme was heat-shock-activated, supporting the concept that trehalase activity can be also enhanced in cells by another mechanism in which cAMP does not act as second messenger.