Sulfate conjugation is one of the major catabolic processes involved in the inactivation and elimination of a variety of phenolic drugs and other structurally related exogenous agents. Sulfation of pharmacologically active xenobiotics is assumed to promote their biological inactivation and, subsequently, facilitate excretion of these agents in the kidney by increasing their water solubility. What has generally been overlooked and disregarded as of major physiological consequence is the fact that a variety of endogenous bioactive substances can also be sulfated in vivo, although in many cases the significance of these reactions has not been appropriately evaluated. Included in the group of endogenous compounds that are known to undergo extensive sulfate conjugation are the catecholamine neurotransmitters dopamine (DA), norepinephrine (noradrenaline, NA), and epinephrine (adrenaline, A) (Roth and Rivett, 1982 ), as well as several neuropeptides including enkephalin (Unsworth and Hughes, 1982 ) and cholecystokinin (Dockray et al., 1978 ). As will be pointed out in greater detail below, sulfation of these endogenous compounds requires a common sulfate donor, 3′-phosphoadenosine-5′-phosphosulfate (PAPS), but is catalyzed by distinct forms of the enzyme phenol sulfotransferase (PST).