The treatment of rats with ciprofibrate, a potent peroxisome proliferator, led to increased levels of the peroxisomal acyl-CoA oxidase (ACO) mRNA. How ciprofibrate functions to elevate ACO mRNA is not known. To help determine the mechanism of ciprofibrate action, in vitro transcription assays were performed. It was determined that ciprofibrate was responsible for a 3.5-fold stimulation of the rate of ACO transcription within 24 hr of ingestion. It was also observed that the transcription rate stimulation following a 2-week ciprofibrate treatment of Wistar rats was maintained following 4 weeks of ciprofibrate withdrawal. Re-introduction of the drug after the 4-week pause resulted in greater stimulation than was initially observed. The results demonstrate that the effect of ciprofibrate is rapid and persists at least twice as long as the initial treatment period. In Zucker rats, both lean and obese, ACO mRNA levels were examined following 2 weeks of ciprofibrate treatment (1 or 3 mg/kg body weight/day). The presence of increased blood levels of triglycerides did not increase ciprofibrate action on transcription, although basal levels of transcription of peroxisomal enzymes were higher in obese rats. The increase in the ACO mRNA level was greater than the transcription rate stimulation suggesting a post-transcriptional regulation.