The [3H]-prazosin binding activity of membranes isolated from aerobically perfused, ischaemic and reperfused cat and rat hearts was investigated. Alpha 1 adrenoceptors in membranes from aerobically perfused cat hearts had a KD of 0.363 +/- 0.067 nM and a Bmax of 68.8 +/- 7.0 fmol/mg protein; 30 min normothermic global ischaemia caused an increase (P less than 0.01) in density (Bmax, 111.4 +/- 9.3 fmol/mg protein), without any change in affinity (KD 0.430 +/- 0.069 nM). Post-ischaemic reperfusion (15 min) caused the Bmax to return to control values, with no change in KD. Membranes isolated from aerobically perfused rat hearts contained a single population of high affinity alpha 1 adrenoceptor binding sites, with a KD of 0.092 +/- 0.02 nM and a Bmax of 43.02 +/- 2.49 fmol/mg protein. Neither global nor low-flow (0.1 ml/min) ischaemia for either 30 or 60 min, nor post-ischaemic reperfusion, caused any change in either the affinity or density of these binding sites. In both species, and under all the above experimental conditions, the selectivity of the alpha 1 adrenoceptor binding sites was maintained, with phentolamine much greater than rauwolscine in displacing bound [3H]-prazosin. These results show that the ability of ischaemia to increase alpha 1 adrenoceptor density in cardiac membranes is species specific, and that it can occur as a direct (as opposed to a reflex) response to ischaemia.