Previous studies within Dow Corning have shown that repeated inhalation exposure to OctamethyIcyclotetrasiloxane (D4) produces reversible hepatomegaly and dose-dependent induction of hepatic microsomal CYP2B1/2 enzymes. Maximum induction of CYP2B1/2 was achieved between 5 and 7 days of exposure. However, a detailed dose-response curve for the induction of CYP2B1/2 has not been determined. Moreover, the amount of D4 in the liver, blood, and fat over a wide exposure range has not been shown. Therefore, the objective of this study was to provide a complete dose-response profile for the induction of CYP2B1/2 activity in rat hepatic microsomes that can be related to exposure concentration and tissue dose.