Melatonin assay has proved to be clinically useful for the diagnosis and investigation of several diseases (e.g., Cushing's syndrome, depressive disorders). We have evaluated the analytical performance of a radioimmunoassay for the determination of melatonin in plasma. The interference of some anticoagulants has been investigated, as well as the preliminary extraction of the hormone using a liquid-liquid and a solid-phase extraction method. The variation coefficients of the assay, within and between runs, were between 3.8% and 9.2% and between 4.1% and 10.5%, respectively. The well-documented circadian rhythm of melatonin secretion was confirmed in our healthy subjects. Preliminary results appear to confirm the significance of melatonin measurement in neurosurgical patients.