Quinidine is the drug of choice for the treatment of cardiac arrhythmias in horses. The plasma concentrations vs. time profiles following oral administration of two formulations of quinidine sulphate, an oral solution and an oral suspension paste, were evaluated in nine horses. They received multiple administrations of the oral solution under fed and non-fed conditions and of the paste under non-fed conditions. A loading dose of 20 mg.kg-1 and a maintenance dose of 10 mg.kg-1 quinidine with dosing interval of 6 h were used. The relative bioavailability of the oral solution under fed conditions in comparison to the solution under non-fed conditions was 75.0 +/- 10.2% for the loading dose and 97.18 +/- 31.66% after the fourth dose. For the paste formulation the relative bioavailability values are not reported, as steady-state levels were not reached. There was a large variation in plasma quinidine levels when the paste formulation was administered. Feeding conditions had a significant influence on the Cmax values after administration of the loading dose. The Tmax values were not affected by food intake. It was concluded that an oral solution has to be preferred because of the variable drug bioavailability from the paste formulation and the poor acceptability of the paste by the horse.