The urine-ethanol concentration (UEC), the urine-methanol concentration (UMC) and the ratio of serotonin metabolites, 5-hydroxytryptophol (5HTOL) to 5-hydroxyindoleacetic acid (5HIAA), were determined in two successive voids from apprehended drunk drivers (n = 35). The blood-ethanol concentration (BEC) ranged from 0-3.00 g/l (mean 1.87 g/l, median 2.03 g/l) compared with 0-3.96 g/l (mean 2.48 g/l, median 2.73 g/l) in the first urinary void and 0-3.56 g/l (mean 2.24 g/l, median 2.47 g/l) in the second void. The UEC decreased significantly from 2.48 +/- 0.99 g/l to 2.24 +/- 0.95 g/l (mean +/- S.D.) between first and second voids as did the UEC/BEC ratios, changing from 1.33 +/- 0.15 to 1.20 +/- 0.10. The BEC and UEC were highly correlated; r = 0.97 +/- 0.04 (p < 0.001) for the first void and r = 0.98 +/- 0.03 (p < 0.001) for the second void. The UMC increased from 7.51 +/- 4.95 mg/l to 8.01 +/- 5.04 mg/l between the first and second voids and the mean difference of 0.50 +/- 0.78 mg/l was statistically highly significant (p < 0.001). The ratios of 5HTOL/5HIAA were 771 +/- 363 pmol/nmol and 728 +/- 377 pmol/nmol in first and second voids, respectively and the difference was not statistically significant (p > 0.05). Finding raised concentrations of methanol and a high 5HTOL/5HIAA ratio in urine specimens provides additional evidence to confirm recent drinking. These biochemical markers might prove useful whenever the integrity of blood or urine specimens is questioned, for example, owing to contamination with extraneous ethanol during collection or microbial synthesis of ethanol in vitro after sampling.