Soybean [Glycine max (L.) Merr.] is grown in many countries and climates. Twenty-two North American soybean cultivars from six different maturity groups were grown from seed under the same conditions. Measurement of metabolic heat rate of leaf tissue with a scanning calorimeter revealed that the slope of heat rate versus temperature when plotted on Arrhenius axes showed abrupt changes reflecting changes in metabolism. The chilling response temperature for all cultivars was near 17.5 degrees C, The maximum tolerable temperature for all cultivars was near 43.5 degrees C, Differences in response to temperatures between the extremes relate to maturity group, follow latitudinal trends, and represent adaptation to different climates. Smaller differences were also observed between cultivars in the same maturity group. Selection of cultivars of soybean for best growth in different climates has resulted in relatively rapid adaptation to local temperatures.