A gas phase mini-circulation apparatus was made and used to release hydrogen cyanide from test blood (1ml) containing cyanide (CN^-) by addition of 3 ml of 13.3% phosphoric acid (final concentration : 10%) at 70°C ; the hydrogen cyanide was then absorbed with cooled 0.1 N sodium hydroxide solution (5 ml) during gas-phase circulation for 20 min. The amount of cyanide absorbed by the 0.1 N sodium hydroxide solution was determined by a modified pyridine-pyrazolone method at 50°C to accelerate the color development. The minimal detectable quantity of cyanide by the proposed colorimetry was 0.1 μg/ml blood. A linear relationship was observed between the absorbance and the amount of cyanide in blood within the range of 0.1-2.5 μg (3.8-96.1 nmol). The recovery was 97.9% and the precision was 2.0% (n=10) when blood (1 ml) containing cyanide (1.2 μg/ml) was used as the test sample. The present method required only 1 h for determination of cyanide in 1 ml of blood. The concentrations of cyanide in blood samples from 5 persons who had died from cyanide poisoning and from 9 who had succumbed in fires were determined by the present method. Significant amounts of cyanide (2.0-22.8 μg/ml, 76.9-876.2 nmol/l) were present in the majority of victims of cyanide poisoning in contrast with the much lower amounts (0-4.9 μg/ml, 0-188.3 nmol/l) in fire victims.