MV‐678 [1‐(8‐methoxy‐4,8‐dimethynonyl)‐4‐(1‐methylethyl)benzene], a recently developed insect growth regulator, increased the hepatic cytochrome P‐450‐dependent monooxygenase enzymes that metabolize endogenous and exogenous chemicals. In an initial set of experiments, male and female rats received 0, 50, or 800 mg/kg·d of MV‐678 by gavage for 3 d, and in a second set of experiments, male rats received 0, 50, or 800 mg/kg·d of MV‐678 by gavage for 30 d. A significant incrase in both absolute and relative liver weight, microsomal protein content, cytochrome P‐450 content, NADPH‐cytochrome P‐450 reductase activity, and ehtylmorphine N‐demethylase activity was observed in male and female rats at the high dose level at 3 d. Similar increases were observed in the 800‐mg/kg·d males at 30 d. Hepatocellular hypertrophy and proliferation of endoplasmic reticulum observed at both 3 and 30 d correspond to and was consistent with microsomal enzyme induction. Reversibility of both induction and changes in morphology was determined by measuring the same parameters in animals treated for 30 d after a 15‐ or 30‐d recovery period. At 15 d recovery, all biochemical parameters at the high dose level, except relative liver weight and microsomal ethylmorphine N‐demethylase activity, had returned to control levels. No significant differences between the control and high dose group animals were noted at 30 d recovery. The hepatocellular changes observed in the high‐dose group at 30 d were less apparent at 15 d recovery, and absent at 30 d recovery.