The field desorption mass spectral behaviour of benzo[a]pyrene metabolites were studied. Each of the phenol, diol, quinone and diol-epoxide metabolites of benzo[a]pyrene showed a single peak corresponding to the molecular ion. The water soluble glucuronides and sulfate conjugated metabolites showed protonated and cationized molecular ions. With the glucuronides, three peaks were observed and the abundance of these peaks may be indicators for characterization of the three isomeric glucuronides.