Quinidine is effective for treatment of atrial fibrillation (AF) in horses, but often accelerates ventricular response rate. Diltiazem effectively controls heart rate response to AF in other species. This investigation determined the effects of diltiazem on cardiac rate and rhythm, left ventricular (LV) function, central hemodynamics, and peripheral blood flow in normal, standing, nonsedated horses. A dose-finding study was performed. Afterward, 8 healthy horses were treated with diltiazem IV every 30 minutes to achieve cumulative dosages of 0 (saline control), 1, 1.5, and 2 mg/kg. Plasma diltiazem concentration, heart rate and rhythm (by electrocardiography), LV function and central hemodynamics (by cardiac catheterization), LV dimensions (by echocardiography), and forelimb blood flow (by Doppler sonography) were determined during each treatment period. Diltiazem plasma concentrations between 390 and 910 ng/mL were achieved, with considerable variation among horses. Cardiac effects of diltiazem included intermittent depression of the sinus and atrioventricular (AV) nodes and mild impairment of systolic and diastolic LV function. Vascular effects of diltiazem included arterial vasodilatation, increased limb blood flow, and decreased systemic vascular resistance. Baroreceptor reflex-mediated sympathetic activation increased sinus node rate and presumably blunted the depressive effects of diltiazem on myocardial and nodal tissues. Two horses developed transient high-grade sinus arrest with severe systemic hypotension. Diltiazem appears relatively safe in healthy horses, but dosage may be limited by hypotension from vasodilatation and direct suppression of sinus node discharge. Because of its inhibitory effects on AV nodal conduction, diltiazem may prove useful for heart rate control in horses with AF.