A specific and sensitive method for the determination of cyclazocine and norcyclazocine in the human serum and urine was developed. Trifluoroacetyl (TFA) derivatives of cyclazocine and norcyclazocine were analyzed by mass fragmentography by using pentazocine as an internal standard. Selected ions of TFA derivatives of cyclazocine and norcyclazocine were m/z 352 and 269, respectively. The result of recovery experiments with the urine sample fortified with the known amount of cyclazocine was satisfactory. The radioimmunoassay was also developed for cyclazocine analysis and time course study of serum concentration after intramuscular administration of 300 μg of cyclazocine was performed. Cyclazocine levels in the human urine samples were determined simultaneously by gas chromatography-mass spectrometry and radioimmunoassay. The coefficient of correlation was 0.97. The total excretions of cyclazocine in three human urines for 48 hr were 30.8%, 31.0% and 47.3%.