Maximum concentrations of guanabenz hydrochloride in plasma (1.2 to 5.2 ng/ml) reached 2 to 5 hours after administration of capsules containing 16 or 32 mg of 14C-labelled guanabenz hydrochloride. Guanabenz hydrochloride was 90% bound to human plasma proteins. Guanabenz hydrochloride competed for imidazoline I2-binding sites in brown adipose tissue with Ki of 97 nM.