The effect of N-hydroxy-2-acetylaminofluorene on the ultrastructure and synthesis of hepatic neclear membranes was evaluated in partially hepatectomized rats. The incorporation of L-[4,5-3H]leucine into two nuclear membrane fractions increased within 2 hr after hepatic resection and reached a peak at 20 hr. After partial hepatectomy, the decay of radioactivity in nuclear membrane proteins labeled with L-[4,5-3H]leucine revealed similar half-lives for the two membrane fractions when compared to those obtained from sham-operated animals. The protein concentration of the nuclear membrane fraction of higher density decreased sharply within 2 hr after partial hepatectomy and remained low throughout a 20-hr postoperative period. Polyacrylamide gel electrophoresis of both nuclear membrane fractions showed a similar composition. Nine proteins were resolved, varying from 21,000 to 190,000 daltons. The two major protein bands were in the range of 50,000 and 70,000 daltons, respectively. Treatment of partially hepatectomized animals with N-hydroxy-2-acetylaminofluorene showed marked dilation of the nuclear envelope and rough endoplasmic reticulum in situ upon electron microscopic examination. Vacuolization and evagination of the perinuclear membranes were also noticeable in isolated nuclei obtained from carcinogen-treated rats. Inhibition by N-hydroxy-2-acetylaminofluorene of the incorporation of L-[4,5-3H]leucine into the nuclear membranes was dose-dependent and remained depressed throughout a 60-min labeling period. These results suggest that the inhibitory effects on RNA and protein synthesis previously shown to be produced by this arylhydroxylamine hepatocarcinogen may lead to disruption of the morphology and synthesis of the nuclear envelope.