The essential fatty acid arachidonate is oxidized by cytochrome P-450 epoxygenases to four epoxyeicosatrienoic acids (EETs): 14,15-, 11,12-, 8,9-, and 5,6-EETs. Each of the four EET regioisomers and their hydrolysis products (DHETs) has multiple paracrine and autocrine functions and may also potently dilate blood vessels and activate potassium channels. The present work describes a method to resolve EETs and DHETs by capillary electrophoresis (CE) using trimethyl-beta-cyclodextrin and CH3CN as buffer additives. While stored at 25 degrees C, most of the EET and DHET regioisomers remained intact when suspended in alkaline vehicle. However, under these same conditions, 5,6-EET rapidly broke down to a lactone and was slowly converted to 5,6-DHET. When subjected to CE, the EET and DHET regioisomers were baseline resolved (R > or = 1.3); 10 pg of an EET or a DHET regioisomer was readily detectable at 194 nm. In addition, the UV spectra were regiospecific and identical to those obtained during HPLC except that an additional, weak absorption occurred at 235 nm. Together, the high-sensitivity, high-resolution, and differential UV spectra permitted the identification and quantification of EETs in phospholipids isolated from murine liver. Thus, CE was successfully used for the trace analysis of eicosanoids.