The quantitative determination and accurate mass measurement of five tricyclic amine pharmaceutical drugs (doxepin, desipramine, imipramine, amitriptyline and trimipramine) fortified in human plasma within a per sample run time of 18 s was accomplished by atmospheric pressure ionization (API) time-of-flight (TOF) mass spectrometry using a turboIonspray liquid chromatography/mass spectrometry (LC/MS) interface coupled with high-performance liquid chromatography (HPLC). The relatively short HPLC separation (18 s) was achieved using a short C 18 column (15 ร 2.1 mm i.d.) with a high aqueous mobile phase maintained at a flow-rate of 1.4 ml min -1 . An acquisition speed of 0.2 s per spectrum accommodates these fast separation conditions. This method employs a one-step liquid-liquid extraction procedure to isolate the five tricyclic amines from biological matrix components The overall extraction recovery was 75% for desipramine and >90% for the other four tricyclic amines. The lower level of quantitation was 1-2 ng ml -1 for each compound. The calibration curve was linear from 2 to 100 ng ml -1 for desipramine and from 1 to 50 ng ml -1 for the other four tricyclic amines. A deuterated internal standard, imipramine-d 3 , was used for all five tricyclic amines. Acceptable intra-and interassay precision (1.0-17.7%) and accuracy (0.2-14.5%) were obtained. The linear dynamic range was extended to 200 based on a software upgrade for correcting ion current detection saturation. The accurate masses of the five tricyclic amines were determined by on-line LC/TOFMS analyses of biological extracts using two-point internal mass calibration. This was done by infusing a reference standard, Jeffamine D230, post-column into the HPLC effluent. All results showed a mass error not greater than 9 ppm for all the target compounds. These results were obtained from both synthetic mixtures when as little as 100 pg were injected or extracts of spiked human plasma samples with analytical concentration as low as 5 ng ml -1 . The factors influencing accurate mass measurements are discussed.