Male Wistar rats exposed intermittently to cyclopentanone vapour at 50, 100 or 300 p.p.m. 6 hrs daily 5 days a week for 1 to 15 weeks showed brain and perirenal fat solvent concentrations which correlated to the exposure although they decreased at the later stages of the experiment. Cyclopentanone caused almost no effects on liver drug metabolizing enzymes or aldehyde dehydrogenase activities while an early increase in the kidney 7-ethoxycoumarin O-deethylase was found after 1 week without a concomitant increase in the cytochrome P-450 content. This effect was abolished later so that an enhancement was found only at the highest exposure level after 15 weeks. Postmitochondrial fractions isolated from liver of exposed animals metabolized cyclopentanone more actively than controls after 15 weeks.