1. The aim of the present study was to examine the effects of isoteoline (IST), a new tetrahydroaporphine derivative, on the blood pressure and some of the components of renin-angiotensin-aldosterone system (RAAS) in spontaneously hypertensive rats (SHR). 2. After long-term treatment (14 days, 3 times daily) in a dose of 1 mg/kg orally and intraperitoneally, IST induced a significant decrease of the systolic blood pressure (SBP) in SHR (from 192 +/- 2 to 155 +/- 5 mgHg). 3. At the same time, IST induced a significant decrease of the levels of plasma renin activity (PRA) and aldosterone (ALD) and manifested a trend to increase the sodium plasma level. 4. These results indicate that IST after chronic long-term treatment induced a marked antihypertensive effect and inhibited the components of RAAS in SHR. Apparently, the long-lasting antihypertensive effect of IST in SHR was mediated and realized in part by inhibition of the components of RAAS.