It has been postulated that Mg depletion is associated with decreased melatonin. Exogenous magnesium (Mg) has been found to increase the activity of serotonin N-acetyltransferase, an enzyme in the pathway for melatonin synthesis; but no data have been found on the effect of Mg deficiency on plasma melatonin. This pilot study examined the effect of a dietary Mg deficiency on plasma melatonin in male, Sprague-Dawley rats. Weanling rats were placed on a Mg-deficient (150 ppm) or a Mg-adequate (1000 ppm) diets for four weeks, after which they were sacrificed 4, 5 or 7 hours into the dark cycle. Plasma was assayed for melatonin concentrations. A significant decrease (p = 0.0101) occurred in mean (+/- SEM) plasma melatonin levels of the Mg-deficient animals (50 +/- 6.4 pg/mL) when compared to the Mg-adequate animals (75 +/- 6.6 pg/mL). There was no obvious phase shift in the melatonin profile of the Mg-deficient animals when compared to the Mg-adequate animals.