Trehalose content and thermotolerance were closely correlated in wild type yeast (Saccharomyces cerevisiae) and in cyr1-2 and bcy1-1 mutants both during exponential growth at 27 degrees C and during heat shock at 40 degrees C. Trehalose levels were high when heat shock proteins (hsps) were expected to be induced and low when hsps were presumably absent. It was tried to uncouple trehalose biosynthesis and hsp-induction. Various non-heat stresses affected trehalose levels of wild type cells in a similar way as they would have affected hsps. However, no trehalose was accumulated when cells were treated with canavanine, a well-known inducer of hsps but not of the thermotolerant state.