Elevated CO2 levels are hypothesized to play a role in the initiation and maintenance of estivation in snails through disturbances of acid-base status. The aim of our study was to identify the ambient CO 2 threshold that induces disturbances in acid-base status in the air-breathing land snail Helix lucorum. Acid-base parameters were determined in the hemolymph of snails acclimated to 0.5%, 1%, 2%, 4%, and 8% CO2 in air for 20 d. In addition, we evaluated the effects of long-term acclimation on metabolic rate and on levels of D-lactate dehydrogenase activity (D-LDH) and of D-laclate in snails after 20 d of exposure to increased CO2 levels. Helix lucorum proved to be unable to compensate for a decrease in extracellular pH (pHc) when acclimated to levels higher than 1% CO2 in air. The rate of oxygen consumption started to decrease when snails were acclimated to 0.5% CO2 in air. However, there was no correlation between the drops in pHc and in metabolic rate. Long-term acclimation to elevated CO2 levels induced an increase in the activity of D-LDH with a concomitant accumulation of D-lactate in tissues. This indicates that long-term acclimation to elevated ambient CO2 levels could reduce the aerobic capacity of land snails and trigger expression of anaerobic pathways of ATP turnover. The threshold levels of ambient CO2 that induce changes in acid-base status and elicit metabolic depression in adult land snails H. lucorum are higher than the future atmospheric levels that are expected to result from human use of fossil energy resources. © 2007 by The University of Chicago. All rights reserved.