The involvement of cytosolic Ca2+ 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 Mn2+ and EGTA, suggesting that the elevation of cytosolic Ca2+ by the ionophore caused phytoalexin production. The effects of various inhibitors of Ca2+-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, LaCl3, 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 Ca2+ is involved in the induction of the plant responses by elicitors.