The extent of oxygen release from two heme proteins, haemoglobin and myoglobin have been studied in the presence of trifluoperazine and chlorpromazine (5-1000 μM). At a molar ratio (drug:protein) of 1.5, the release of oxygen from haemoglobin was 4 and 15% in the presence of chlorpromazine and trifluoperazine respectively, while from myoglobin the corresponding values were 20 and 40%. The findings were attributed to the greater extent of local conformational change around tryptophan moieties of each of the proteins induced by trifluoperazine.