Pooled plasma from healthy volunteers was spiked with pure, synthetic chlorpromazine (CPZ), chlorpromazine sulfoxide (CPZSO), or chlorpromazine N-oxide (CPZNO), and then made alkaline with either sodium hydroxide or sodium carbonate. The samples were allowed to stand at room temperature for various timed intervals before extraction with organic solvent. It was found that CPZNO was reduced to CPZ in plasma made alkaline with sodium hydroxide, but not in protein-free buffer solution at high pH nor in plasma made alkaline with sodium carbonate. The reaction appears to take place through reducing equivalents generated by the action of sodium hydroxide on plasma proteins. Thus, apparent concentrations of CPZ in plasma from patients were elevated by as much as 343% when sodium hydroxide was used compared with concentrations in aliquots of the same plasma samples alkalinized with sodium carbonate. The amount of CPZ produced from CPZNO depends on the type of extraction procedure employed as well as on the quantity of sodium hydroxide added to the plasma. By contrast, no interconversion between CPZ and CPZSO or CPZNO and CPZSO was observed in plasma alkalinized and extracted under any of the conditions tested.