1. The stereochemistry of mandelic acid, produced as a major urinary metabolite of ethylbenzene and styrene in rat and man has been investigated. Although these solvents are both achiral they are metabolized to chiral metabolites, via a series of chiral intermediates. 2. Analytical methods (g.l.c.-mass spectrometry, h.p.l.c. and 19F-n.m.r.) have been developed for the determination of the enantiomeric composition of mandelic acid in urine. 3. These methods have been applied to the study of the metabolic stereochemistry of ethylbenzene and styrene in rats dosed orally (100 mg/kg body weight) and in human volunteers exposed to atmospheres containing these solvents at the upper limits prescribed for workplaces by the UK Health and Safety Executive (100 ppm in air). 4. Results show that whereas only the R-enantiomer of mandelic acid was excreted after ethylbenzene exposure, the mandelic acid formed from styrene was essentially racemic. In three workers occupationally exposed to styrene, ratios of R to S isomers of 1.16, 1.27 and 1.14 were found. A synthetic R/S mixture of mandelic acid had an R/S ratio of 1.03. 5. The implications of these findings for the biological monitoring of workers occupationally exposed to stryrene and/or ethylbenzene are discussed.