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Spontaneously hypertensive rats were given an angiotensin-converting enzyme (ACE) inhibitor (benazepril or quinapril) or hydralazine and were left for up to 6 hr. To examine whether administration of antihypertensive agents affects expression of immediate early genes in left ventricular myocardium, groups of rats were sacrificed at 1,3, and 6 hr after dosing; total RNA was extracted from left ventricular tissue and analyzed by blot hybridization technique using labeled probes for c-myc, c-fos, and GAPDH mRNA. All three antihypertensive agents reduced pressure similarly, and treatment with the two ACE inhibitors increased c-fosand c-mycmRNA expression in left ventriculum. By contrast, hydralazine did not increase steady-state mRNA expression of either proto-oncogene. Thus, in parallel with the pressure fall, acute administration of the ACE inhibitors induced expression of c-fosand c-mycmRNAs in the left ventricle. Since the equidepressor dose of hydralazine did not affect expression of these proto-oncogenes, this effect of ACE inhibitors is independent of their hemodynamic action. [P.S.E.B.M. 1995, Vol 210]