OBJECTIVE: To assess whether reduction of heart rate (HR) has beneficial effects on endothelial function in patients with type 2 diabetes mellitus (T2DM). DESIGN: Randomised, double-blind, placebo-controlled study. SETTING: University hospital. PATIENTS: 66 T2DM patients without overt cardiovascular disease. INTERVENTIONS: Patients were randomised to receive for 4 weeks, in addition to their standard therapy, one of the following treatments: atenolol (25 mg twice daily), ivabradine (5 mg twice daily) or placebo (1 tablet twice daily). MAIN OUTCOME MEASURES: Systemic endothelial function, assessed by flow-mediated dilation (FMD); endothelium-independent vasodilation, assessed by nitrate-mediated dilation (NMD); cardiac autonomic function, assessed by HR variability (HRV). RESULTS: 61 patients completed the study (19, 22 and 20 patients in atenolol, ivabradine and placebo groups, respectively). Compared with baseline, HR was similarly reduced by atenolol (87+/-13 vs 69+/-9 bpm) and ivabradine (86+/-12 to 71+/-9 bpm), but not by placebo (82+/-10 vs 81+/-9 bpm) (p<0.001). FMD improved at follow-up in the atenolol group (4.8+/-1.7 vs 6.4+/-1.9%), but not in the ivabradine group (5.2+/-2.5 vs 4.9+/-2.2%) and in the placebo group (4.8+/-1.5 vs 4.7+/-1.7%) (p<0.01). NMD did not change significantly in any group. HRV parameters did not change in the placebo group; they, instead, consistently increased in the atenolol, whereas a mild increase in SDNNi was only observed in the ivabradine group. A significant correlation was found in the atenolol group between HR and FMD changes (r=-0.48; p=0.04). CONCLUSIONS: Despite a comparable reduction in HR, atenolol, but not ivabradine, improved FMD in T2DM patients suggesting that changes in HR are by themselves unlikely to significantly improve endothelial function.