Pooled plasma from healthy volunteers was spiked with pure, synthetic clozapine or clozapine N-oxide and then made alkaline with either sodium hydroxide or sodium carbonate. The alkalized samples were allowed to stand at room temperature for various time intervals before extraction with organic solvent and then analyzed by high-performance liquid chromatography. It was found that clozapine N-oxide was reduced to clozapine in plasma made alkaline with sodium hydroxide, but such reduction was negligible in the plasma made alkaline with sodium carbonate. The amount of clozapine produced from clozapine N-oxide depended upon both the strength of the alkali added to the plasma and the duration of exposure of the plasma proteins plus clozapine N-oxide to the alkali. The reduction appears to take place through reducing equivalents generated by the action of strong alkali on plasma proteins. The thermal lability of clozapine N-oxide during gas chromatography-mass spectrometric analysis was also investigated. It was found that clozapine N-oxide was quantitatively decomposed to clozapine during gas chromatography-mass spectrometric analysis.