We studied the effects of two P-glycoprotein (P-gp) inducers, 2-acetylaminofluorene (2-AAF) and phenothiazine (PTZ), administered intraperitoneally, on the activities and content of hepatic cytochrome P-450 (CYP) subfamilies in hepatic microsomes of Sprague-Dawley rats. After 4-day administration of 2-AAF or PTZ, the P-gp content was increased. The total CYP content after PTZ treatment was significantly increased compared with that of controls. The CYP1A, CYP2B and CYP3A2 contents were induced, while the CYP2C6, CYP2C11 and CYP2E1 contents remained unaffected. A marked increase in CYP1A1 was found after administration of each compound. Ethoxyresorufin O-deethylase, pentoxyresorufin O-deethylase, and testosterone 6beta hydroxylation activities showed a significant increase after both 2-AAF and PTZ treatments. In particular, ethoxyresorufin O-deethylase exhibited more than ten times greater activity than that of the controls after the treatments. These results suggest that P-gp inducers affect several CYP subfamilies in addition to CYP3A, which is reported to be up-regulated coordinately with P-gp by a CYP3A inducer.