1. The present study investigates the effect of short-term experimental diabetes of 14-days duration on the β-adrenoceptor subtypes of the rat heart. 2. β-adrenoceptor-mediated functional responses to submaximal doses of isoprenaline were enhanced in Langendorff-perfused hearts from diabetic rats, manifested as greater changes in tension, heart rate and rates of tension development (+dT/dt) and decline (-dT/dt). 3. Radioligand binding data demonstrated that total cardiac β-adrenoceptor density and affinity for [3H]-dihydroalprenolol was unchanged by diabetes, although a decrease in β1-adrenoceptor density and increase in β2-adrenoceptor density was observed. 4. In conclusion, hearts from 14-day streptozotocin-induced diabetic rats demonstrate a number of alterations within the β-adrenoceptor system. However, the enhanced β-adrenoceptor-mediated responses to isoprenaline were not caused by an overall increase in density of β-adrenoceptors, but were accompanied by changes in the ratio of the β-adrenoceptor subtypes.