Extracellular ATP is a known receptor agonist in animals and was previously shown to alter plant growth, and so we investigated whether ATP derivatives could function outside plant cells as signaling agents. Signaling responses induced by exogenous nucleotides in animal cells typically include increases in free cytoplasmic calcium concentration ([Ca(2+)](cyt)). We have evaluated the ability of exogenously applied adenosine 5'-[gamma-thio]triphosphate (ATPgammaS), adenosine 5'-[beta-thio]diphosphate (ADPbetaS), and adenosine 5'-O-thiomonophosphate to alter [Ca(2+)](cyt) in intact apoaequorin transgenic Arabidopsis thaliana seedlings. ATPgammaS and ADPbetaS increase [Ca(2+)](cyt), and this increase is enhanced further when the nucleotides are added with the elicitor oligogalacturonic acid. Exogenous treatment with ATP also increases the level of transcripts encoding mitogen-activated protein kinases and proteins involved in ethylene biosynthesis and signal transduction. The increase in [Ca(2+)](cyt) induced by nucleotide derivatives can be ablated by Ca(2+)-channel blocking agents and by the calcium chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), and the changes in gene expression can be partially blocked by these agents. These observations suggest that extracellular ATP can activate calcium-mediated cell-signaling pathways in plants, potentially playing a physiological role in transducing stress and wound responses.