Ketorolac is a potent nonsteroidal anti-inflammatory drug. Recently, a novel ester of ketorolac, ketorolac pentyl ester, was synthesized. When prepared in injectable oil, the new agent demonstrated a long duration of action. Ketorolac pentyl ester was synthesized using a prodrug design by esterification of ketorolac, and appeared to be a prodrug of ketorolac in vivo, which needed to be confirmed. The aim of the present study was to establish the prodrug's pharmacokinetics in vivo, and to confirm whether or not ketorolac pentyl ester was a prodrug of ketorolac. Pharmacokinetic profiles of intravenous ketorolac and its pentyl ester on an equal-molar basis in six rabbits were evaluated. A high-performance liquid chromatographic method was used to determine the plasma concentrations of ketorolac and its pentyl ester. We found that the plasma concentrations of ketorolac pentyl ester declined rapidly after injection and so did the conversion of ketorolac pentyl ester to ketorolac. Also, the conversion of ketorolac was proved complete when compared with intravenous ketorolac under an equi-molar basis. In conclusion, this in vivo pharmacokinetic study confirmed that keterolac pentyl ester was a prodrug of keterolac.