The pharmacokinetic variability of moclobemide, a new short half-life reversible selective inhibitor of monoamine oxidase (MAO) was investigated through analysis of concentrations measured during early open clinical use. Eighty-nine depressed patients, aged 21–96 years, were included in the present study. Doses ranged from 200 to 900 mg/day, and the time interval between blood sampling and last drug intake on the previous day was between 8 and 23 h. Intraindividual variability was generally moderate, with a few patients displaying consistently high concentrations despite moderate doses. Interindividual variability for measured concentrations was ∼300-fold. After concentration decrease with time was taken into account (average half-life estimate of 4.6 h), age was identified as a major factor responsible for between-patient variability. Average concentration increase per decade of age was 38%. Neither gender, weight, height, smoking, nor alcohol intake explained a significant additional part of the variance. Analysis of residuals also suggested that phenytoin co-medication may induce moclobemide metabolism. The present study indicates that concentration monitoring of a newly marketed drug can contribute to gaining insight into its pharmacokinetic behavior and to enhancing its rational use in clinical practice.