We used (H~3)-prazosin on rabbits to determine the changes of ?_1-adrenoceptor number (Bmax) and affinity (k_D) before and after ischemia, Within 15min of ischemia, we demonstrated that Bmax increased to twfold in ischeamic regions. The increase persisted for 30 min of ischemla and then began to fall at 45 min of ischemia. At the same time, in the nonischemic regions Bmax did not change. The results also showed that ischemia did not alter K_D markedly in both nonischeamic and ischeamic regions.