Adult male rats were exposed to 7.9 mumol/1 (300 ppm) of styrene for 2--11 weeks 6 h daily excluding Saturdays and Sundays. The exposures caused a marked styrene accumulation in brain and perinephric fat, and the accumulation tended to increase upto the 4th week. The body styrene content decreased thereafter gradually towards the end of the experiment to reach half of the styrene content of the 4th week. Serum creatine kinase activity increased at the initial stage of the exposures while serum non-specific cholinesterase was below the control range at the same time. Earliest biochemical changes were detected at the 9th week of exposure, and they included increased activity of lysosomal acid proteinase. Increased enzyme activity prevailed to the end of the exposures. Simultaneous minor alterions could be detected in spinal cord axonal protein pattern whereas water-soluble protein composition in cerebellum did not change. Biochemical alterations in brain were not accompanied by significant changes in serum enzyme activities. It is concluded that marked metabolic adaptation to inhaled styrene takes place. Serum enzyme determinations may prove valuable in the adaptation period whereas they may not reflect beginning chronically neurotoxic effects.