To clarify whether alpha 1-adrenergic receptors contribute to cardiac hypertrophy, we examined alpha 1-adrenergic receptor densities in the right and left ventricles of 5- and 20-week-old spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY) using [3H]prazosin. Also, we analyzed a pharmacologically distinct subtype of alpha 1-adrenergic receptors by pretreatment with chloroethylclonidine. The ratio of right ventricular weight to body weight as well as left ventricular weight to body weight was higher in SHR than in WKY at 5 weeks of age. At 20 weeks of age, only the left ventricular weight to body weight ratio was higher in SHR. At 5 weeks of age, the right and left ventricular alpha 1-adrenergic receptor densities in SHR were greater than those in WKY (LV 1760 vs. 1275, RV 2543 vs. 1521 fmol/mg protein). At 20 weeks of age, only right ventricular alpha 1-adrenergic receptor density remained higher in SHR (1425 vs. 885 fmol/mg protein). The percentage of pharmacologically distinct alpha 1A receptor was higher in the right ventricle of SHR than in that of WKY at 5 weeks of age (25% vs. 18%). There were no significant differences in the dissociation constants among the alpha 1-adrenergic receptor assays. These findings suggest that an increase in alpha 1-adrenergic receptors might be involved in cardiac hypertrophy in the early phase of hypertension in SHR.