Injection of Benzo(a)pyrene into rats results in urinary excretion of fluorescent metabolites of BaP. The synchronous fluorescence (SF) technique can detect picogram quantities of fluorescent compounds such as 7,8,9,10-tetrahydro-tetrahydroxy-benzo(a)pyrene (BPT). BPT was used as a model compound to show that SF can detect 0.1μM of polynuclear aromatic compounds (PNA) or PNA metabolites in urine by simple dilution. The relatively high concentration limit is due to the need to dilute the urine 100 fold before the characteristic pyrenyl spectrum is observed. The upper limit of direct quantification of BPT without dilution of whole urine is 5μM because of urine quenching effects. A 100-fold dilution of urine, containing added BPT, restores normal fluorescence and SF spectra. The 0.1 μM detection limit is lower than the observed amounts of individual PNA in nonexposed, nonsmoking humans. This approach may provide a simple, noninvasive tool for monitoring human exposure to fluorescent toxic substances.