We examined the relationship between trehalose metabolism and the leavening ability of bakers' yeast grown in the presence of NaCl. The yeast cells were cultured art 30 degrees C in media (5% glucose, 1% peptone, 6.5% yeast extract, 0.1% KH2PO4, and 0.1% MgSO4) containing 0-3% NaCl. The cells were grown to the stationary phase for 24 h irrespective of the NaCl concentration in the medium, but the cell yield was decreased by addition of NaCl. The presence of 1% NaCl in the medium transiently enhanced the accumulation of intracellular trehalose after 24 h, however, the accumulated trehalose was hydrolyzed during further growth, and 2 or 3% NaCl decreased the intracellular trehalose content. The neutral trehalase activity in yeast cells grown for 24 h increased with increasing NaCl concentration, while the acid trehalase activity decreased. Trehalose-6-phosphate synthase (TPS) activity in cells grown for 24 h increased significantly in the presence of 1 and 2% NaCl, but decreased in the presence of 3% NaCl. When we determined the leavening ability of bakers' yeast cells grown for 24 h in the presence of 0-3% NaCl using dough without addition of NaCl, the leavening ability increased with increasing NaCl concentration in the culture medium. The cells having the highest leavening ability were those that had the lowest amounts of trehalose and the lowest TPS activity among the conditions tested. The leavening ability of bakers' yeast grown in the presence of NaCl appears not to correlate with intracellular trehalose content.