Study object — The aim was to determine whether the changes in β adrenoceptors might be involved in the conduction system of diabetic rat hearts. Design — β Adrenoceptors in the atrioventricular node of diabetic hearts and controls were characterised by quantitative autoradiography. Experimental material — The experiments were performed in five male Wistar rats injected with streptozotocin (65 mg·kg−1) to induce diabetes and in five rats injected with vehicle alone. Measurements and main results — 125I-iodo-cyanopindolol binding sites decreased by 25% in atrioventricular node of diabetic hearts compared to control hearts without a significant change in the affinity. The selective β1 adrenoceptor antagonist CGP20712A (100 nmol·litre−1) displaced about 30% and 50% of the total atrioventricular node binding sites in diabetic and control hearts, respectively. Conclusions — These data support the concept that a decrease in β adrenoceptors, mainly of the β1 subtype, in the atrioventricular node may play an important role in the cardiac conduction disturbance of diabetic animals.