A rapid simple gas chromatographic technique for determining nomifensine in plasma is described. 2. Nomifensine is rapidly absorbed after oral administration to man. 3. Following a 100-mg dose, peak levels of an acid-labile conjugate of 2--3 microgram/ml are reached about 1.5 h after administration. 4. The conjugate is cleared with a half-life of between 1 and 2 hours.