In humans 6-sulphatoxy melatonin (SaMT) is the principal metabolite of endogenous and exogenous melatonin. 5-sulphatoxy N-acetyl-serotonin (SNAS) is a minor metabolite of exogenous melatonin, but it has not been established whether the levels of endogenous SNAS in plasma derives principally from endogenous melatonin. We have developed the first radioimmunoassay (RIA) for SNAS and used it (together with RIAs for melatonin and SaMT) to determine whether endogenous SNAS derives from endogenous melatonin or from platelet serotonin. Our results show a) the values of endogenous SNAS, unlike endogenous SaMT, increased with blood collection procedures that increased the values of serotonin, b) the values of endogenous SNAS in urine or in platelet-poor plasma were approximately the same as those of endogenous SaMT, but, unlike SaMT, did not show a diurnal rhythm, and c) we confirmed that SNAS was a minor metabolite of orally ingested melatonin. Thus, our conclusion is that SNAS is a minor metabolite of exogenous melatonin, but is not a significant metabolite of endogenous melatonin. In all probability, endogenous SNAS is principally the metabolite of platelet serotonin.