A marked increase in the amount and relative proportion of fraction II of the smooth endoplasmic reticulum (SER II) and a concomitant decrease in the SER I fraction were observed in the liver of male Sprague-Dawley rats fed diets containing 0.05% (w/w) 2-acetylaminofluorene (AAF) for various lengths of time (3-18 weeks). The amount of rough endoplasmic reticulum (RER), which remained at control levels after 3 weeks of AAF feeding, i.e. before the appearance of hyperplastic nodules (HPN), was clearly decreased after 18 weeks of continuous AAF-feeding (HPN present). When the rats were subjected to 4 or 5 cycles of interrupted AAF feeding, similar increases in SER II were also observed both in the homogenates of HPN-containing livers, as well as in the homogenates of HPN dissected out from the surrounding liver tissue. SER II, the predominant microsomal membrane fraction in the livers of rats fed AAF for 3 weeks, showed the highest level of induction of epoxide hydrolase. A marked elevation of the manganese-dependent enhancement of polysome binding in vitro was also observed in SER II from the livers of AAF-fed rats.