Cholecystokinin-pancreozymin (CCK-PZ) is involved in the regulation of pancreatic protein synthesis and secretion. We demonstrate here that CCK-PZ also stimulates RNA synthesis. Rats were killed 0, 30, 60, 120, 240 or 480 min after intraperitoneal injection of CCK-PZ (8 U/kg). Nuclei were prepared from pancreata and used for in vitro RNA synthesis ('run-on' experiments) in the presence of [alpha-32P]UTP. Total RNA synthesis increased after CCK-PZ with maximum UTP incorporation at 60 min. Contributions of RNA polymerase II, responsible for mRNA synthesis, and RNA polymerases I and III could be separately estimated by using alpha-amanitin. RNA polymerase I and III activities increased by 68% after CCK-PZ, whereas RNA polymerase II activity increased by 113%. Rates of synthesis of amylase, chymotrypsinogen B, trypsinogen I and actin mRNAs were estimated by quantitative hybridization of newly synthesized transcripts to specific cDNA clones. Synthesis of the four RNAs increased with a maximum between 60 and 120 min after CCK-PZ, stimulation being more important for the serine proteinases than for amylase. It was concluded that, in rat pancreas, CCK-PZ controls gene expression at the transcriptional level.