In order to validate measurement of urinary sulphatoxymelatonin as an accurate method of estimating plasma melatonin secretion in older people, we compared 24 h plasma melatonin secretion and sulphatoxymelatonin excretion with renal function in 20 subjects 62-89 years of age. There was a good correlation between plasma and urinary sulphatoxymelatonin over the same 24 h period (R2 = 0.797) and no relationship between creatinine clearance and sulphatoxymelatonin excretion (R2 = 0.075). The results suggest that sulphatoxymelatonin excretion estimation is a good surrogate measurement of plasma melatonin secretion in older people, at least across the range of creatinine clearance for the subjects in the study, 0.41-1.81 ml/sec.