We studied safety and tolerability of neladenoson bialanate, a novel oral selective partial adenosine A1 receptor agonist that maintains the cardioprotective effects of adenosine without the undesired side effects of a full agonist, in 2 pilot studies in patients with heart failure with reduced ejection fraction (HFrEF). The β-blocker interaction study was a single-blind, placebo-controlled study on the effects of a 30-mg single dose of neladenoson bialanate on atrioventricular (AV) conduction in 11 patients with HFrEF treated with β-blockers. The PARSiFAL pilot study was a double-blind, placebo-controlled study on the effects of a 7-day treatment with 10 or 20 mg neladenoson bialanate or placebo in 31 patients with HFrEF on β-blocker therapy. In the β-blocker interaction study with 11 HFrEF patients, no second- or third-degree AV block was detected on 48-hour Holter monitoring. In the 31 HFrEF patients included in the PARSiFAL pilot study, no second- or third-degree AV blocks were observed during 24-hour Holter monitoring, and no effects were seen on heart rate and blood pressure. Median absolute changes in LVEF, measured by cardiac magnetic resonance, were 1.9% (interquartile range -1.1 to 4.3), 0.3% (-1.4 to 2.7), and 2.2% (0.4 to 4.5), in the placebo, 10-mg, and 20-mg groups, respectively. Treatment of HFrEF patients with the novel partial adenosine A1 agonist neladenoson bialanate appeared to be safe in 2 small pilot studies, and no atrioventricular conduction disorders or neurological side effects were observed. No significant early changes in cardiac function were detected.