The aim of this study was to determine if the endogenous levels of gamma-hydroxybutyric acid (GHB) in urine were affected by drinking and smoking. Urine samples were obtained from 20 healthy volunteers (15 males, 21-45 years; 5 females, 22-24 years). This population included four average drinkers (males), 4 average smokers (males), and 12 nonsmokers/nondrinkers (seven males and five females). Urinary levels of GHB were measured by gas chromatography. No gender differences were observed in the urinary levels of endogenous GHB. The urinary levels of GHB in males were 0.52+/-0.37 microg/ml in smokers, 0.28+/-0.21 microg/ml in nonsmokers/nondrinkers, and 0.23+/-0.04 microg/ml in drinkers. Urinary GHB levels were measured three times a day for 5 consecutive days in a male from each group. Large intra-individual differences were observed over the 5-day period in a smoker and a nonsmoker/nondrinker. No significant changes in daily endogenous GHB levels were observed in a drinker during the period. Our preliminary results suggest that stimulatory effects of nicotine on the central nervous system (CNS) may result in an increase in nocturnal formation of GHB and the depressive effects of ethanol on the CNS may not affect, even may inhibit, nocturnal production of GHB.