In this study, an original surgical implantation technique in the confluens sinuum via the superior sagittal vein was developed to quantify melatonin secretion by the pineal gland. Melatonin (CAS 73-31-4) was determined using gas chromatography couples to negative ion chemical ionisation mass spectrometry following liquid extraction and derivatisation by pentafluoropropionic acid anhydride (PFPA). The minimum detectable amount was 40 fg per injection, corresponding to 1 pg.ml(-1) in dialysate. The assay was linear in the range 20-1000 pg.ml(-1). This method was suitable for routine melatonin determination in dialysats of peripheral and central circulation with coefficients of variation of 11.2 and 24.6%, respectively for within and between analyses. Profiles of melatonin concentration were obtained (n = 3 rats) over a 2-day experimentation with a slowly diminution of the filtration capacity of the probe during the second day. The nocturnal concentrations of melatonin in the confluens sinuum dialysat ranged from 1003.9 to 2345 pg.ml(-1) in the dialysat, indicating wide interindividual variations in the melatonin levels.