alpha 2-Adrenoceptors were studied in renal membrane fractions from spontaneously hypertensive (SHR), two-kidney, one clip hypertensive (2K, 1C HT) and DOCA-salt hypertensive (DOCA-salt HT) rats, using radioligand binding method. alpha 2-Adrenoceptor concentration in the kidney measured by [3H]yohimbine binding was significantly increased in SHR at 4 weeks old (41.5 +/- 2.8 fmol/mg protein, mean +/- SEM, p less than 0.01), 12 weeks old (54.9 +/- 2.5 fmol/mg protein, p less than 0.01) and 35 weeks old (59.8 +/- 3.4 fmol/mg protein, p less than 0.01) as compared with age-matched Wistar-Kyoto rats (WKY, 31.5 +/- 2.5, 40.9 +/- 1.8, 47.8 +/- 2.0 fmol/mg protein, respectively). There were no significant differences in binding affinity and 5'-nucleotidase activity (plasma membrane marker enzyme) between SHR and WKY at any age. In 2K, 1C HT rats, alpha 2-adrenoceptor concentration in the clipped kidney was higher than that of control rats, but alpha 2-adrenoceptor concentration in the unclipped kidney was unchanged. Binding affinity and 5'-nucleotidase activity showed no significant changes in renal hypertensive rats. In DOCA-salt HT rats, no significant change was found in concentration and affinity of renal alpha 2-adrenoceptor. The observed increase in renal alpha 2-adrenoceptor concentration in SHR may contribute to the pathogenesis and maintenance of hypertension through increased sodium and water reabsorption in the kidney.