1. The enzymic hydrolysis of a wide series of nicotinic acid esters was investigated using human and rat plasma, and purified hog liver carboxylesterase, and compared with previously published data from rat liver microsomes. Esterase activities were always found to obey Michaelis-Menten kinetics. 2. Rat liver microsomal and plasma enzyme velocities were six orders of magnitude smaller than those of purified hog liver carboxylesterase, and three orders smaller than human plasma activities, but the Km values were of the same magnitude. 3. The binding of nicotinate esters to human plasma esterases, and purified hog liver carboxylesterase, appears to depend mainly on hydrophobic and steric factors.