The objective of the present study was to determine the relationship between plasma renin levels, and the development of hypertension and cardiac hypertrophy in TGR (mREN2)27 hypertensive rats. Systolic blood pressure and left ventricular mass index (LVMI) were measured in transgenic heterozygote and normotensive Sprague Dawley control rats at 25, 35, 45, 55, 65, and 75 days of age together with determinations of plasma active renin and prorenin, and renal and adrenal tissue renin, which were assayed at pH 6.5, 7.4, and 8.5. The systolic blood pressure and the LVMI of the transgenic rats were significantly increased compared to control rats by 55 and 65 days of age, respectively. Plasma active renin of the transgenic rats, measured at physiological pH, was significantly higher from 55 days of age, increasing in parallel with blood pressure and remaining significantly higher than controls at all age groups tested. Assays of both plasma and adrenal renin at various pHs showed a profile of angiotensin I generation that matched mouse renin more closely than that of rat renin. The ratio of angiotensin I (Ang I) generation at pH 8.5 and pH 6.5 was 0.5 for normal rat plasma but was between 3 and 5 for mouse plasma. Plasma prorenin and adrenal tissue renin from transgenic rats exhibited a pH profile consistent with the major portion being mouse renin. However, the low level of kidney renin observed in the transgenic rats exhibited a pH ratio (8.5/6.5) identical to that of normal rat renin (0.5), suggesting that residual renin within the kidney was predominantly of rat origin. These data indicate that plasma renin levels closely parallel the development of high blood pressure and LVMI and show that interpretation of the renin status of this strain is critically dependent on the assay conditions used. Under the conditions used in this study it was found that the TGR(mRen2)27 rat is a high mouse plasma renin model of hypertension.