The involvement of cytosolic Ca 2+ in the signal transduction mechanism of phytoalexin induction was examined in oats. The treatment with a calcium ionophore, A23187, strongly induced the accumulation of avenanthramide A, a major phytoalexin in oats, in a dose-dependent manner. The induction of avenanthramide A by A23187 was suppressed by Mn 2+ and EGTA, suggesting that the elevation of cytosolic Ca 2+ by the ionophore caused phytoalexin production. The effects of various inhibitors of Ca 2+-dependent processes on the elicitor activity of penta- N-acetylchitopentaose ((GlcNAc) 5) were also examined. Treatment of oat leaf segments with three calcium channel blockers, LaCl 3, verapamil and nifedipine inhibited the phytoalexin induction by (GlcNAc) 5 at sub-millimolar levels. TMB-8 and ruthenium red, inhibitors of intracellular calcium movement, also abolished the phytoalexin induction. These findings indicate that Ca 2+ is involved in the induction of the plant responses by elicitors.