Jasmonic acid can be assayed by the highly sensitive and reproducible gas chromatography-negative ion chemical ionization-mass spectrometry (GC-NICI-MS) technique. We describe an optimized sample preparation method for routine analysis of jasmonic acids which allows the analysis of more than 10 samples/day. The basis of the method resides in a one-step extraction and phase partition, selective adsorption/elution of acids with an aminopropyl column, conversion of acids to pentafluorobenzyl esters, final purification using a silica column, and GC-NICI-MS. Extracted standard curves were linear over an assay range of 1 to 1000 ng; correlation coefficients were typically greater than 0.9999. The limit of detection was 500 fg (2.4 fmol) jasmonic acid/injection at a signal to noise ratio of 10:1. Furthermore, the method was modified for steric analysis of jasmonic acid stereoisomers at the nanogram level. (3R,7S)-(+)-7-Iso-jasmonic acid was found to be biosynthesized from plant tissue cell cultures upon elicitation. This stereoisomer epimerizes rapidly under alkaline and acidic conditions and in the presence of albumin. Epimerization rate constants were determined at different pH values.