Mirtazapine is the most widely prescribed tricyclic anti-depressant pharmaceutical product. The aim of the studywasthe development of optimal conditions for the isolationand analysis of Mirtazapine from biological fluids. It is nec-essary to develop rapid, high-sensitivity and specific analy-sis methods that facilitate the chemical-toxicological analy-sis of mirtazapine. Was developed a rapid and effective (95%)optimal conditions of liquid-liquid extraction for isolation ofMirtazapine from human plasma: organic solvent system -Ethanol: Isoamyl alcohol: Dimethyl sulfoxide (90: 5: 5), dou-ble extraction for 10 minutes. Developed an optimal condi-tion of liquid chromatographic- tandem mass spectrometricmethod of qualitative-quantitative analysis of Mirtazapinein human blood plasma: the mobile phase-0,1% formic acid :Acetonitrile - 0.1% formic acid : water (80:20), system iso-cratic, column temperature - 35 oC, collision energy 22 ev,ionization positive (ESI +), multi-reaction monitoring mode(MRM). Developed method was validity on some: Linearity(caliber concentration 10.0 - 100.0 ng / ml, correlation coeffi-cient – R2 = 0.9982), selectivity; low limit of detection (LOD -10.09 ng / ml) and limit of quantification (LOQ - 24.77 ng / ml),precision and accuracy.