Gamma-hydroxybutyric acid's (GHB's) natural presence in the body has made the interpretation of its levels a challenging task for the forensic toxicologist. This study was designed to measure endogenous GHB levels in antemortem urine and blood samples. The range detected in urine was from 34 to 575 microg/dl and in blood from 17 to 151microg/dl. The results indicate that the concentration of endogenous GHB in urine and blood concur with the suggested cut-off levels at 1000 and 500 microg/dl, respectively.