Specific binding of (3H)-prazosin and (3H)-yohimbine to plasma membrane vesicles of mesenteric arteries from SHR and WKY rats at 16-18 weeks was compared. The radioligand binding was carried out in Mg-MOPS buffer at 25 degrees C for 25 min. There was no change in the density or the affinity of (3H)-prazosin binding in SHR vessels (Bmax 188 +/- 33 fM/mg protein, Kd 1.26 nM) compared with WKY (Bmax 211 +/- 28 fM/mg protein, kd 1.24 nM). However, the Bmax of (3H)-yohimbine binding was moderately, but significantly, increased in SHR arteries (Bmax 323 +/- 22 fM/mg protein) compared with WKY (Bmax 252 +/- 17 fM/mg protein) without any change in the sensitivity (Kd 26 nM versus 25.9 nM). These results suggest that postsynaptic alpha 2-adrenoceptors density is increased in hypertensive rat mesenteric arteries. The role of this increased number of receptors in the pathogenesis of hypertension is not known.