Thermospray mass spectrometry has proven to be a useful technique for analyzing various biological compounds including eicosanoids phospholipids. Molecular ions as well as fragment ions which reveal useful structural information are produced for underivatized eicosanoids phospholipids using filament-off or filament-on thermospray mass spectrometry, respectively. In conjunction with on-line chromatographic separation, complex mixtures of biological samples can be rapidly analyzed with great reliability. Data will be presented concerning the analysis of prostaglandins other eicosanoids molecular species of phospholipids as well as the application of these methodologies to complex biological samples.