A flow injection configuration for the spectrofluorimetric determination of magnesium is proposed. The method is based on the formation of a strongly fluorescent complex of magnesium ion with 2-hydroxy-1-naphthaldehyde salicyloylhydrazone in alkaline, aqueous-ethanolic (50% v/v) solutions. The optimal chemical conditions for complex formation in the mixed solvent were studied and the flow injection manifold was optimized and used for the determination of magnesium in 1000-fold diluted serum samples without any other pre-treatment. The measurement throughput was 120 h-1. The detection limit of the method was 2 micrograms l-1 (2 mg l-1 for serum). The concentration range of application is 4.8-120 micrograms l-1 (0.2-5.0 mumol l-1). Within-run relative standard deviations for the method at 4.8, 24 and 120 micrograms l-1 of magnesium were 3.5, 1.2 and 0.7%, respectively. Analytical recoveries range from 96.2 to 102% (mean 98.8%).