A rapid and sensitive method using matrix-assisted laser desorption ionization (MALDI)-quadrupole time-of-flight (QTOF)-mass spectrometry (MS) was developed for the analysis of six tricyclic antidepressants (ADs) and their related drugs, such as amitriptyline, carbamazepine, clomipramine, imipramine, nortriptyline, quetiapine, and two tetracyclic ADs, mianserin and mirtazapine, because these eight drugs are commonly observed medicines in poisoning cases in Japan. In the present MALDI-MS method, the limits of detection were 0.3-2 ng/ mL. The present method provided a simple and high throughput simultaneous analysis of these drugs. This is the first report dealing with quantitation of ADs and their related drugs in biological samples by MALDI-MS. In addition, we applied the MALDI-MS to two authentic cases, and three new metabolites of amitriptyline and three new metabolites of quetiapine could be detected in blood and urine samples in the two cases when regioisomers were counted as one metabolite.