N-acetyl-N-formyl-5-methoxykynuramine (AFMK) is a melatonin metabolite identified in rat brain by Hirata et al.(The Journal of Biological Chemistry 249 (1974) 1311). Since no assay has been described for its routine measurement, we have developed and validated such a radioimmunoassay. We synthesized AFMK and N-acetyl-5-methoxykynuramine (AMK), in order to produce anti-AFMK antibodies and to standardize the assay. The tracer [3H]- ...