Soybean [Glycine max (L.) cv. Jack] grown in open top chambers under controlled laboratory and field conditions was ušed to study the acclimation of leaf gas exchange processes to CO2 enrichment. Air inside the open top chambers was maintained at either 700-800 or 350-400 pmol(C02) mol'^(air). Leaf gas exchange rates were measured for some plants switched between treatments. When measmed in the C02-emiched atmosphere, stomatal conductances (gg) were higher in leaves grown in C02-enriched atmospheres than in those grown under ambient conditions, and the lower gg values for plants in the C02-enriched atmospheres were limiting to leaf net photosynthetic CO2 exchange rates (Pn). of enriched leaves was higher than those of the ambient Controls when measured at elevated CO2 levels in both controlled environment and field studies, while it was depressed in enriched leaves when measured imder ambient CO2 conditions, and this drop in Pn did not recover until 6-15 d after plants were placed back in ambient conditions.