The octadecanoid pathway is responsible for producing jasmonic acid an important signaling molecule in plants, which controls the production of a variety of secondary metabolites. Previously the exogenous addition of jasmonic acid to Catharanthus roseus hairy roots caused an increase in terpenoid indole alkaloid (TIA), accumulation. The role of the endogenous production of jasmonic acid by the octadecanoid pathway in the production of TIAs in C roseus hairy roots is examined. Feeding of octadecanoid pathway inhibitors suggests that the octadecanoid pathway does not actively control TIA production under normal growth conditions or during the UV-B stress response in C roseus hairy roots. Biotechnol. Bioeng. 2009;103: 1248-1254. (C) 2009 Wiley Periodicals, Inc.