When [14C]haloperidol decanoate, an ester of haloperidol and decanoic acid, was given intramuscularly to rats, levels of total radioactivity and haloperidol decanoate in medial iliac and hypogastric sacral lymph nodes nearest to injection sites were the highest in examined lymph nodes and plasma. These lymph node levels became maximum 16 days after administration and declined gradually with half-life (around 14 days) similar to those of plasma total radioactivity, haloperidol decanoate and haloperidol. However, when the labelled ester was given intravenously, plasma total radioactivity disappeared far more rapidly. Much more radioactivity was found in hind limbs whose femoral muscles had been injected than in other body parts, even at late stages after administration. Haloperidol alone was found in the brain after [14C]haloperidol decanoate was given either intramuscularly or intravenously. It was concluded that haloperidol decanoate injected in rat femoral muscle was rate-limitedly distributed in lymph circulation and that the absorbed ester did not penetrate the brain through the blood-brain barrier but formed haloperidol did.