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Atrial fibrillation (AF) is considered the most common arrhythmia affecting exercise performance in horses. Quinidine is an effective therapy for conversion of AF to sinus rhythm, but treatment is associated with a number of adverse clinical effects. Furthermore, residual electrical and mechanical abnormalities of the equine atria may account for the recurrence of AF following successful therapy. Relative to these issues, we sought to better understand the physiology of the in vivo equine atria and the associated sinoatrial and atrioventricular (AV) nodal tissues. In treating horses with AF using the drug quinidine, a common adverse effect is marked acceleration of the ventricular response rate prompting early discontinuation of treatment. Diltiazem effectively controls ventricular response to AF in other species, but has not been evaluated in horses. In our first series of experiments we studied the pharmacodynamic and pharmacokinetic effects of the calcium channel blocker diltiazem on the cardiovascular system in healthy horses in sinus rhythm. To further support these studies we characterized the clinical electrophysiology of the normal equine atria and then quantified the electrophysiologic effects of quinidine and diltiazem in a pacing model of atrial tachycardia. Based on our studies demonstrating inhibitory effects on AV nodal conduction, diltiazem is likely to be useful for ventricular rate control in horses with naturally occurring AF undergoing quinidine treatment. Diltiazem appeared relatively safe in healthy horses, but dosage may be limited by hypotension from vasodilatation and direct suppression of sinus node discharge. In our last two studies, we established echocardiographic techniques to assess mechanical function of the equine left atrium noninvasively and applied these methods to a subset of horses recently converted from AF to normal sinus rhythm. Our preliminary guidelines for echocardiographic assessment of LA size and mechanical function will likely be useful to study LA function in horses with cardiac disease. Specifically, we were able to show that LA mechanical function in horses can be significantly depressed after successful conversion of AF to sinus rhythm. The clinical relevance of these findings will have to be established in future investigations.