The autoradiographic localization of alpha 1- and alpha 2-adrenoceptors was identified in 22-week-old Wistar-Kyoto rat kidney, using alpha 1-selective ([3H]bunazosin) and alpha 2-selective ([3H]yohimbine) antagonists. [3H]Bunazosin binding was distributed predominantly over the cortex, less over the outer medulla and was absent from the inner medulla. [3H]Yohimbine binding was distributed predominantly over the medulla, less over the renal cortex and was absent from the inner medulla. In addition, the distribution of renal alpha-adrenoceptors was investigated in 3-, 7- and 22-week-old spontaneously hypertensive rats (SHRs) using a computerized image analysis system. Renal alpha-adrenoceptors were both found to be increased in SHRs at all ages tested compared with their respective controls and were increased in both the cortex and the outer medulla. The increase in renal alpha-adrenoceptors was already present in 3-week-old SHRs whose systolic blood pressures did not differ significantly from those of the controls. The results strongly suggest that these abnormalities of renal alpha-adrenoceptors may play a critical role in the development of hypertension in SHRs.