Rat liver monooxygenase activities, such as androstenedione (AD)-hydroxylase, pentoxyresorufin (PR)- and ethoxyresorufin (ER)-O-deethylases have been studied under contact application of ultrasound (US). In the livers of US-treated rats the AD-hydroxylase activity did not differ from the control, thus suggesting that CYP2A1, CYP2B1, CYP2C11, CYP3A1 were not induced. The rate of ER-deethylation increased from 100 in control up to 300 pmol/min/mg in US-treated rats. Western blot analysis revealed also the CYP1A1 induction in US-treated rats which was absent in untreated animals. The mRNA level for CYP1A1 in the livers of US-treated rats increased from 0.5 up to 6.0 fmol mRNA/total RNA but did not change for CYP1A2. The results obtained provide evidence for the nonchemical induction of CYP4501A1 which is known to be induced by polycyclic aromatic compounds as well as by some other compounds.