For the assay of the antidepressant compound mianserin hydrochloride (Org GB 94) in human plasma, a mass fragmentographic method, using the deuterated analogue as internal standard and a high-performance liquid chromatographie sample clean-up procedure has been developed. The assay specifications obtained are a lower limit for reliable measurements of 1 ng/ml, and accuracy of ca. 0.01 ng/ml, a precision of 6--7% and a capacity of about 60 samples per day. The applicability of the assay method is illustrated by measurements of single-dose and steady-state plasma levels in clinical experiments, demonstrating the possibility of monitoring plasma levels during at least 24 h after a single dose of 15 mg of Org GB 94. The mean steady-state plasma levels after a daily dose of 3 X 20 mg of Org GB 94 appeared to be remarkably constant with time: 38, 36 and 34 ng/ml after 2, 4, and 6 weeks of treatment of 18 depressed patients.