When [14C]haloperidol decanoate, a long-acting neuroleptic and an ester of haloperidol and decanoic acid, was incubated in human whole blood and plasma and in rat plasma and homogenates of rat brain, lung, liver, kidney, pancreas and muscle, no hydrolysis of the ester was seen. Although the decanoate was hydrolyzed by partially purified carboxylesterase, addition of rat plasma or liver homogenate to the enzymic reaction mixture resulted in marked inhibition of hydrolysis, whereas addition of the defatted residues of plasma or liver produced only partial inhibition. The enzymic hydrolysis was inhibited also by beta-lipoprotein and albumin, depending on their concentrations. The assumption that interaction between haloperidol decanoate and protein resulted in inhibition of the hydrolytic reaction mediated by the enzyme was validated by kinetic models and experimental data. The kinetics were apparently competitive. Based on the kinetic analysis, the interaction between the decanoate and albumin or beta-lipoprotein was investigated by measuring their equilibrium constants and extent of protein binding. Haloperidol decanoate appeared to interact with several proteins; this was exemplified by other measures of protein binding, an increasing effect of proteins on the solubility, and the partition ratio of the ester. The interaction between haloperidol decanoate and proteins caused marked stabilization of this ester against enzymatic hydrolysis and, thereby, influenced its metabolism.