The expression of renin mRNA was determined by a quantitative polymerase chain reaction assay in 27 human kidney samples: (1) 15 biopsies of patients with glomerulonephritis with or without angiotensin-converting enzyme inhibitor (ACEI) treatment; (2) biopsies of six renal allografts with graft rejection; and (3) six biopsy samples from unaffected parts of tumor nephrectomy specimens as controls. After isolation of RNA, 0.5 to 1 microgram of total RNA was used for reverse transcription to generate cDNA. The human renin gene was subsequently amplified by the use of two primers spanning the second and third exons. Renin expression was quantified with a renin cDNA mutant as the internal standard. It exhibited the same primer binding sites as the endogenous gene but carried a 155-basepair deletion, thus yielding a shorter amplification product. The number of glomeruli was counted by microscopic transillumination immediately after biopsy (median, 9 per biopsy; range, 2 to 23). Renin mRNA was expressed as femtograms of renin mRNA per glomerulus. Renin gene expression was lower in glomerulonephritic patients without ACEI treatment compared with that in control tumor nephrectomy samples, i.e., 63 +/- 20 (N = 7) versus 250 +/- 50 fg (N = 6) of renin mRNA/glomerulus, (P < 0.02), although plasma renin concentration in the glomerulonephritic patients was in the normal range. Significantly higher renin mRNA expression was found in glomerulonephritic patients treated with ACEI, i.e., 210 +/- 50 (N = 8) compared with 63 +/- 20 (N = 7) fg of renin mRNA/glomerulus in patient not treated with ACEI (P < 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)