N-15 NMR analysis reveals alanine production in Duckweed plants exposed to low intensity sinusoidally varying magnetic fields (SVMF) at 60 and 100 Hz, and fed by N-15-labeled ammonium chloride. Alanine does not accumulate in the absence of SVMF. Addition of vitamin C, a radical scavenger, reduced alanine production by 82%, indicating the roll of free radicals in the process. Alanine accumulation in plants and animals in response to exposure to a variety of stress conditions, including SVMF, is a general phenomenon. It is proposed that alanine is a universal first stress signal expressed by cells. (C) 2003 Elsevier Science (USA). All rights reserved.