AbstractThe monooxygenase activity of fetal hepatocytes in culture shows a differential response toward juvenile hormone I and analogs. Juvenile hormone I, R‐20458, and Methoprene increase the deethyiation of 7‐ethoxyresorufin while not affecting or even inhibiting the N‐demethylation of p‐chloro‐N‐methylaniline. RO‐203600, a 1,3‐benzodioxole‐containing analog, increases both the deethylase and the N‐demethylase, whereas Hydroprene does not affect either activity. The inductive effect with juvenile hormone I is obtained with exposure periods of at least 30 min and is maximum when the concentration of the hormone is 14 μM in the medium. This amount results in the covalent binding to cellular macromolecules of 1.3 × 19−18moles/cell. The induction requires continuous protein synthesis but RNA synthesis only for a short initial period. It is concluded that juvenile hormone and mimics induce specific cytochrome P‐450 species in fetal liver cells even if the culture conditions are not optimal. The toxicological implications of these resu