Cardiac hypertrophy accompanies the progressive rise in blood pressure in spontaneously hypertensive rats. The role of endogenous RNA polymerases in this process was examined in nuclei from isolated cardiac myocytes of 20-week-old spontaneously hypertensive rats and normotensive Wistar-Kyoto controls. Both template-engaged (involved in transcription) and free (loosely attached to endogenous template, transcribing only with exogenous templates) RNA polymerases were increased in spontaneously hypertensive rats. In addition, the ratio of RNA polymerases I/II was lower in the spontaneously hypertensive rats for both functional pools of the enzyme. Endogenous transcribing RNA polymerases were quantified by t.l.c. of RNA-hydrolysis products. Increased numbers of enzyme molecules were present in nuclei from spontaneously hypertensive rats, without appreciable change in the rate of polyribonucleotide-chain elongation. These results could not be explained by differences in the activities of contaminating phosphatases or ribonucleases, nor by changes in endogenous nucleoside pools or recoveries of labelled nucleosides. Enhanced myocardial RNA synthesis in the spontaneously hypertensive rats at the stage of established cardiac hypertrophy is associated with increased numbers of RNA polymerase molecules. This increase may, in turn, reflect altered chromatin structure, resulting in increased polymerase binding and/or chain initiation.