Dimethylcyclosiloxanes (DMCS) are components of silicone gel containing implants and are known inducers of human drug metabolizing enzymes. The effects of the major DMCS, octamethyltetracyclosiloxane (D4) on cytochrome P450 (CYP) induction were examined in young adult, mature, and pregnant female Sprague-Dawley rats. Also, the ability of D4 administered to pregnant dams to affect CYP expression in fetal liver was examined. Female young, mature, and pregnant Sprague-Dawley rats were administered 0, 5, 20, and 100 mg/kg D4 daily by gavage for 8 days. Liver microsomal CYP (CYP2B, CYP3A, CYP1A) concentrations were evaluated by Western blots using specific antisera, and CYP activities were assayed using CYP selective assays. D4 treatment resulted in a significant induction of CYP2B and CYP3A isoforms. CYP induction was dose and age dependent. A comparison of the inducibility of CYP3A protein by D4 in rats from different age groups showed that the degree of increase was the highest in the pregnant rats at doses of 20 mg/kg D4 or higher. The mature rats had a lesser degree of responsiveness than did the young rats at the dose of 100 mg/ kg D4. Significant increases in CYP2B immunoreactive protein concentrations were observed in young and mature rats given D4 at doses >5 mg/kg and in pregnant rats at doses >20 mg/kg. Maximal CYP2B induction detected with blotting was more than 90-fold in mature rats; however, no significant changes were detected in CYP1A expression. There was a 20% increase of liver to body weight ratio in the mature rats treated with 100 mg/kg D4. D4 has different inductive properties in female rats of different ages and reproductive status. Also, D4 administered to the pregnant dam is capable of inducing CYP expression in fetal liver as well as decreasing fetal body weight.