High-performance liquid chromatography/thermospray mass spectrometry (HPLC/MS) was evaluated and optimized for the determination of beta- and gamma-endorphin. Thermospray spectra for the endorphins could be acquired under ion evaporation conditions, when a low vaporizor temperature (94 degrees C) and high source temperature are used. The spectra showed [M + H]+, [M + Hx-1 + Nax]+(x = 1-3) together with double charged ions. Positive ion detection exhibited higher ion currents than negative ion detection. Thermospray HPLC/MS was used to generate a linear calibration curve from 10 to 300 pmol. Thermospray displayed good run-to-run reproducibility (2-7%) and accurately measured spiked quantities of beta- and gamma-endorphin within 15% of the spiked values.