The importance of the inflammatory process in the pathology of experimental Mg-deficiency has been reconsidered but the sequence of events leading to inflammatory response remains unclear. In this study, the effect of Mg-deficiency on complement system by measuring total C3 concentration, mRNA abundance for rat pre-pro complement C3 in liver by RT-PCR, complement haemolytic activity and C3 activation by Western Blot was studied. Weaning male Wistar rats were fed either Mg-deficient or control experimental diets for 2 or 8 days. At 8 days, a characteristic inflammatory response of Mg-deficiency including hyperaemia, leukocytosis and enlarged spleen was accompanied by an increase in the total C3 quantity in plasma. Moreover, at 8 days, RT-PCR analysis indicated higher level of mRNA rat pre-pro complement C3 in liver from Mg-deficient rats compared to control rats. Even if the inflammatory syndrome was not observed in rats after 2 days, total plasma C3 was shown to be significantly increased as compared to total plasma C3 level in control rats. Because of the high variability of complement haemolytic activity values in Wistar rats, weaning male Sprague-Dawley rats were used in a second experiment. At 8 days, the inflammatory response of Sprague-Dawley rats was accompanied by an increase in total C3 quantity and by a higher haemolytic activity. The Western Blot technique failed to display distinct bands resulting from C3 cleavage in plasma from Mg-deficient rats. Since, the complement C3 is a positive acute phase reactant, the elevation of C3 indicates that the modification of inflammatory response is an early event of Mg-deficiency. However, complement activation does not appear to be involved in the acute phase of the deficiency.