Constitutive protein mRNA is utilized as an index for target protein mRNA expression in experimental models. It is assumed that constitutive protein mRNA expression is not altered by disease states or pharmacologic intervention. We evaluated whether pressure overload, or long term angiotensin converting enzyme (ACE) inhibition, would alter the expression of two constitutive protein mRNA's: cyclophilin and G3PDH. 16 wk spontaneously hypertensive rats (SHR) were treated with enalapril (10 mg/kg; n = 9), or placebo (n = 15), for 12 wks, compared to Wistar Kyoto normal controls (n = 18). Terminal hemodynamics were performed following therapy, with aortic flow (mean: MAF, mlfmin). pressure (mean: MAp, mmHg), and heart rate (HR, bpm). Systemic arterial resistance (SAR, units) was derived from digitized waveforms. Myocardial function was assessed by left ventricular end diastolic pressure (LVEDP, mmHg), and contractility (+dp/dt). Hemodynamics were followed by harvest of left ventricle (LV), and kidney (K) tissue, which were stored at –80°C. Total RNA was extracted, and dot blot analysis of cyclophilin and G3PDH mRNA was performed. Each probe mRNA was quantified by densitometry, and expressed as a relative density ratio of cyclophilin/G3PDH (C/G). Renal tissue ACE levels (activity units) were utilized to document ACE inhibition, and were 2.5 ± 1.3 (normal). 1.8 ± 0.8 (placebo SHR). and 1.0 ± 0.3 (enalapril SHR, p = 0.05, compared to placebo). Analysis of variance was performed and mean values are as given below: HR MAP MAF SAR LVED +DP/DT C/G-LV C/G-K Placebo SHR 410 149 † 108 † 1.47 † 9.4 † 16812 † 1.43 1.60 Enalapril SHR 407 92 145 0.67 2.3 8933 1.78 138 Normal 299 80 181 0.46 1.7 12341 1.73 1.52 † P < 0.05: placebo vs enalapril Thus, severe pressure overload does not alter LV and K cyclophilin and G3PDH mRNA expression in the SHR, although age-dependent changes of mRNA expression were not evaluated in this study. Furthermore, long term ACE inhibition did not alter their expression in a systematic manner.