Vascular hypertrophy, renin and blood pressure in the young spontaneously hypertensive rat. 1. Cardiovascular reactivity, blood vessel morphology, blood pressure and the activity of the renin-angiotensin system were determined in the 3-week-old spontaneously hypertensive (SHR), Wistar-Kyoto (WKY) and outbred Wistar (WIS) rat. 2. In an isolated perfused mesenteric artery preparation the SHR had a significantly increased maximum response to KCl and noradrenaline (P less than 0.02) compared with the WKY. Using a myograph, vascular structure was measured over a range of resistance arteries and showed a significant correlation between lumen diameter and both media cross-sectional area and thickness, with the regression line for the SHR shifted upwards indicating both increased media area and thickness. This was associated with a slight, but significant, narrowing of the lumen (P less than 0.01) and an increased media/lumen ratio (0.049 +/- 0.01, 0.034 +/- 0.007, 0.036 +/- 0.008 for SHR, WKY and WIS, respectively, means +/- SD P less than 0.001). The SHR had a greater heart/body weight ratio than either the WKY or the WIS (P less than 0.001). 3. Both mesenteric artery and membrane protein content were higher in the SHR, indicating an increase in cell size or number. 4. Plasma renin activity (means +/- SD) was lower in the SHR (1.0 +/- 0.7 pmol of angiotensin I h-1 ml-1) than in the WKY (2.2 +/- 1.2 pmol of angiotensin I h-1 ml-1, P less than 0.001) but not different from that in the WIS (1.2 +/- 0.8 pmol of angiotensin I h-1 ml-1). Mesenteric artery vascular renin concentration was also lower in the SHR (P = 0.06). 