Procaine added to whole equine blood or diluted plasma was hydrolyzed with half times of approximately 9 and 12 minutes, respectively, at 37 C. This hydrolytic activity was sensitive to heating and physostigmine, but did not affect procainamide. At pharmacologic concentrations of procaine, the rate of the hydrolytic reaction depended directly on the concentrations of plasma or procaine in the system and was less in whole blood than in plasma. These properties are consistent with hydrolysis being due to plasma esterases operating at less than saturating procaine concentrations. These esterases were also inhibited cooling, sodium fluoride, or arsenite. Synovial fluid had approximately 20% of the procaine esterase activity of plasma. Comparison of hydrolytic activities of plasmas from Thoroughbred, Standardbred, and other breeds of horses showed statistically significant differences in the rates at which individual plasmas hydrolyzed procaine. A frequency distribution of these rates showed unimodal distribution, indicating that all horses tested may be regarded as members of a single population.