The analysis of allene oxide synthase (AOS) mRNA levels, of AOS polypeptide levels and specific enzymatic activities, as well as the quantitative determination of the levels of the octadecanoids cis-12-oxophytodienoic acid (cis-OPDA) and JA following a number of treatments, has shown that AOS is a regulatory site in octadecanoid biosynthesis in A. thaliana. AOS activity, mRNA and polypeptide levels are increased in wounded leaves locally and systemically. The methyl esters of OPDA or JA (OPDAME, JAME) and coronatine, are strong inducers of AOS mRNA, polypeptide and enzymatic activity. Ethephon also induces AOS activity. Salicylic acid (SA) was an inducer of AOS activity while abscisic acid (ABA) had no effect. At the level of the octadecanoids, the consequences of induction of AOS by the different inducers were distinctly different, depending on the nature of the inducer. Wounding led to a strong, bi-phasic accumulation of JA in wounded leaves and to a less pronounced increase in JA-levels in systemic leaves. Levels of OPDA changed very little in wounded leaves and remained constant or even declined in systemic leaves. Ethephon treatment resulted in a strong, transient increase in JA-levels kinetically coinciding with the second, more pronounced peak in wound-induced JA. In SA-treated leaves, the level of cis-OPDA increased throughout the experimental period while there was no effect on JA levels during the first 24 h following treatment and only a slight accumulation after 48 h. Clearly, mechanisms in addition to regulating substrate (LA) availability and the regulation of AOS accumulation control the output of the octadecanoid pathway.