1. We have examined the catalytic activities (7-ethoxyresorufin O-deethylase [EROD] and 7-methoxyresorufin O-demethylase [MROD]), protein levels (Western blot analysis) and mRNA levels (Northern blot analysis) of cytochrome P4501A (CYP1A1 and CYP1A2) in the lung, liver and kidney following a single 2.5 mg/kg (15.4 micromol/kg) subcutaneous dose of nicotine to the female Sprague-Dawley rat. 2. Only in lung microsomes was EROD activity significantly induced by nicotine treatment. The activity increased 4.4-fold at 6 h after treatment relative to controls, peaked at 12 h at 14.7-fold the control activity and returned to near control level at 24 h. 3. In parallel with EROD activity, CYP1A1 immunoreactive protein abundance was altered significantly by nicotine treatment only in the lung, peaking at 12 h and decreasing towards control levels thereafter. 4. Following subcutaneous nicotine treatment, CYP1A1 mRNA was detectable in the lung at 6 and 12 h but not at 24 h, was slightly elevated in the kidney at 12 h and was detectable in the liver only at the 12-h point. CYP1A2 immunoreactive protein and its mRNA were detectable only in the liver, and their levels were not affected significantly by nicotine pretreatment. 5. Nicotine affected the binding of Hepa 1c1c7 cytosolic protein to a CYP1A1 xenobiotic response element in a gel mobility shift assay, suggesting involvement of the aryl hydrocarbon receptor and transcriptional activation in CYP1A1 induction by the chemical. 6. Inhaled nicotine also induced pulmonary EROD activity, and the induction by either inhaled or injected nicotine was more pronounced in the male than in the female rat. 7. The findings show that nicotine is a potent, rapid but transient inducer of CYP1A1 in the rat lung and suggest that the alkaloid is a likely contributor to CYP1A1 induction by cigarette smoke.