Exposure of plant cells and tissues to low or freezing temperatures often lead to uncontrolled and detrimental ion leakage. Therefore, when plants acclimate to low temperatures, processes that control ionic homeostasis are important. Here we characterized H + ATPase and ATP-dependent Ca 2+ transport activities in isolated plasma membranes of cold-acclimated and non-acclimated winter rye leaves (Secale cereale L. cv. Voima). Cold acclimation resulted in a two-fold higher Ca 2+ transport activity, significantly different (P = 0.021) from that of non-acclimated rye, whereas only a small increase in H + ATPase activity, measured as ATP hydrolysis, was observed in cold-acclimated compared to non-acclimated preparations. In plasma membranes, extensively washed with EDTA and Brij 58 to remove endogenous calmodulin, Ca 2+ transport activity increased to about double by calmodulin addition, with both non-acclimated and cold-acclimated material. Uptake of Ca 2+ was seen within the pH range analyzed (pH 6-8), with an optimum at pH 7.2 with both materials, and both in the absence and in the presence of calmodulin. The increase in activity of ATP-dependent Ca 2+ transport in cold-acclimated rye plasma membranes probably reflects the capacity needed to sustain the resting level of cytosolic Ca 2+ concentration that is characteristic to the cold-acclimated situation. Β© Elsevier, Paris Calmodulin / Ca 2+ transport / cold acclimation / H + ATPase / plasma membrane / rye / Secale cereale BTP, 1,3-bis[tris(hydroxymethyl)-methylamino]-propane / CaM, calmodulin / CCCP, carbonylcyanide m-chlorophenyl hydrazone / DMSO, dimethylsulfoxide / DTE, dithioerythritol / DTT, dithiothreitol / EDTA, (ethylenedinitrilo)tetraacetic acid / EGTA, ethylenebis(oxyethylenenitrilo)tetraacetic acid / HEPES, N-(2-hydroxyethyl)piperazine-N'-2-ethane sulfonic acid / MES, 2-(N-morpholino)ethane sulfonic acid / MOPS, 3-(N-morpholinopropane)sulfonic acid