Methylphenidate (MPH) is an immediate-release (IR) or sustained-release (SR) drug used to treat attention-deficit hyperactivity disorder. Eight dogs were randomly assigned to two treatment groups, using an open, single-dose, two-treatment, two-period, randomized, crossover design. Each subject received a single dose of 20 mg d,l-MPH IR or SR tablet. After blood collections at specific times, the concentrations of d,l-MPH in plasma were evaluated by high-performance liquid chromatography. Following both IR and SR oral administration of d,l-MPH, the animals did not show any side effects, except that mild hyperkinesia was observed in a few subjects belonging to the IR treatment group. After both administrations, the concentration data for d,l-MPH in plasma displayed a characteristic, one-compartment drug model. The relative bioavailability of the SR formulation was 30.58 +/- 13.73%. Significant differences between the two administrations were found in T(max), C(max), AUC, and Cl. Despite low drug concentrations in the blood, the SR formulation ensured uniformity of d,l-MPH plasma concentrations and, thus, a simpler and easier titration. In conclusion, the tested dosage appears to be too low for clinical application in canines, and an increase in dosing is suggested. Further pharmacodynamics studies are necessary to support this speculation.