[en] A radioimmunoassay of urinary 6-sulphatoxymelatonin (a-MT6s) was performed in 90 normal subjects: 44 males and 46 females (17-67 years). Patients treated with betablokers or antidepressants were not included in this study. Urine samples were collected over three periods of time: 7 to 11 p.m., 11 p.m. to 7 a.m., and 7 to 11 a.m. Between 11 p.m. and 7 a.m., the subjects slept in their normal environment and had not ingested alcohol for 24 hours. We searched for a possible relation between urinary a-MT6s excretion (expressed in ng/l/h) and age. From 7 to 11 p.m. and from 7 to 11 a.m. no significant relation could be found. On the contrary, between 11 p.m. and 7 a.m. there was a significant relation indicating decrease of a-MT6s secretion with increasing age. Several linear or non-linear curve patters were tested: Boltzmann sigmoid (1(st), 2(nd), and 3(rd) degree), polynomial curves. The Boltzmann sigmoid showed the best fit judging by the r-squared value (0.152) and the runs test (p=0.64). On this curve the inflection point was located at 53 4 years (SDM, standard deviation of the mean). From 19 to 45 years, the upper sigmoid plateau was located at 1381 91 ng/l/h (SDM). The decrease was found between 45 and 60 years and the lower sigmoid plateau then stabilized at 467 370 ng/l/h (\SDM). In the study group, there was no significant difference between men and women according to the Mann-Withney test. Finally, use of oral contraceptives did not affect urinary a-MT6s (Mann-Withney).