We describe here a direct a radioimmunoassay (RIA) for the determination of 6-sulfatoxymelatonin (aMT6s) in plasma, with iodinated aMT6s as tracer. The aMT6s antiserum was raised in rabbit by immunization with a bovine serum albumin conjugate, giving negligible cross-reactivities for related compounds. The low limit of detection (15 pmol/L) allowed a direct assay that required only a 100-microL plasma sample. Dilutions of plasma and of synthetic aMT6s gave the same parallel response in the RIA. A preliminary study showed a circadian variation in healthy volunteers, with mean concentrations ranging from 52 (at 1600-2100 h) to 378 pmol/L (at 0400 h), whereas this rhythm was abolished in pinealomectomized patients. After administration of melatonin orally, or by infusion, the aMT6s concentrations in plasma concorded with previous data on aMT6s production and pharmacokinetics, with aMT6s being cleared from plasma more slowly than melatonin. This assay should have practical application in the development of new pharmaceutical formulations that minimize the hepatic metabolism of melatonin.