The role of inositol 1,4,5-trisphosphate (IP 3 ) in transducing heat-shock (HS) signals was examined in Arabidopsis. The whole-plant IP 3 level increased within 1 min of HS at 37 o C. After 3 min of HS, the IP 3 level reached a maximum 2.5 fold increase. Using the transgenic Arabidopsis plants that have AtHsp18.2 promoter-b-glucuronidase (GUS) fusion gene, it was found that the level of GUS activity was up-regulated by the addition of caged IP 3 at both non-HS and HS temperatures and was down-regulated by the phospholipase C (PLC) inhibitors {1-[6-((17b-3-Methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-2,5-pyrrolidinedione}(U-73122).The intracellular-free calcium ion concentration ([Ca 2+ ] i ) increased during HS at 37 o C in suspension-cultured Arabidopsis cells expressing apoaequorin. Treatment with U-73122 prevented the increase of [Ca 2+ ] i to some extent. Above results provided primary evidence for the possible involvement of IP 3 in HS signal transduction in higher plants.