BACKGROUND: The study evaluates the circadian rhythm of melatonin in relation to sex and age and identified contemporary alterations in the secreton of some hypophyseal hormones, suggesting that melatonin may exert a modulatory action on the latter. METHODS: The melatonin metabolite (6-hydroxymelatonin sulfate) was assayed in urine samples from 48 subjects of both sexes aged between 25 and 60 years old using the RIA method described by Arendt, modified for the ise of iodine markers. A blood sample was taken from the same subjects to assay hypophyseal hormones. RESULTS: Melatonin secretion does not remain constant over 24 hours in young subjects of both sexes, but instead is rhythmic. This rhythmic secretion is lacking in adults, revealing a daytime pineal secreton that is surprisingly higher than in younger persons. A difference in secretion levels was also found between sexes, a higher melatonin peak in females compared to males. Some young female subjects reveal a contemporary hypersecretion of the somatotropic hormone in line with the falling nocturnal peak of melatonin. Subjects with latent hypothyroidism show a diminished melatonin peak compared to that in euthyroid subjects. CONCLUSIONS: Changes in pineal secretion between the second and sixth decade of life are characterised by the loss of rhythmic secretion, linked not only to the loss of the nocturnal peak but an increased daytime secretion compared to younger subjects. The other finding that emerged from this study was the difference in secretion levels between the sexes. Lastly, we affirm that the pineal gland may exert a modulating influence on the hypophysis.