Activation of tissue-specific gene expression of the components of the renin-angiotensin system (RAS) in humans may play an important role in cardiovascular regulation and pathophysiology. Studies of human tissue RAS expression, however, have been limited by the lack of availability of sufficient amounts of fresh human tissues and a sensitive method for detecting specific mRNAs. To demonstrate the presence of components of local RASs in humans we used the polymerase chain reaction (PCR) after reverse transcription to detect renin- angiotensinogen-, and angiotensin-converting enzyme-mRNA in small quantities of human tissues. Results indicated that all components of the RAS were widely expressed in human organ samples. In order to study changes of gene expression in small tissue samples (e.g., renal biopsies) obtained from patients, we established a competitive PCR assay for quantification of renin, using a 155-basepair deletion mutant of the human renin cDNA as an internal standard. Renin-mRNA concentration was quantitated in the kidney (1.74 +/- 0.2 pg renin/micrograms total RNA), adrenal gland (1.15 +/- 0.15 pg renin/micrograms total RNA), placenta (0.7 +/- 0.1 pg renin/micrograms total RNA), and saphenous vein (0.02 +/- 0.01 pg renin/micrograms total RNA). The method described here may serve as a highly sensitive tool to quantify alterations in gene expression in man under various pathophysiologic conditions. This study should provide the methodological basis for future studies of tissue RAS in human physiology and disease.