The objective of this work was to analyze oxidative metabolism in Mya arenaria. Total Fe content in M. arenaria collected in the German Wadden Sea was 1.9+/-0.7, 0.7+/-0.1 and 0.17+/-0.01 nmol/mg fresh weight (FW), in digestive glands (DG), mantle and gills, respectively. Labile Fe pool, assessed by electronic paramagnetic resonance (EPR), was 146+/-10 pmol/mg FW, and by the fluorescence method employing calcein it was 118+/-9 pmol/mg FW. The lipid radical content in the DG, assessed by EPR, was 27+/-7 pmol/mg FW, and the thiobarbituric reactive substances content amounted to 57+/-8 pmol/mg FW. Ascorbyl radical (A) content, assessed by quantification of EPR signals, was 0.04+/-0.01 pmol/mg FW, and the ascorbate content (AH(-)) was 478+/-12 pmol/mg FW. The ratio A/AH(-) was (8+/-1)x10(-5)AU, suggesting a minimum oxidative stress even under physiological conditions, presumably depending on basal metabolic functions. The content of nitric oxide (NO), assessed by EPR, was 99+/-3 pmol/mg FW. The generation rate of NO by nitric oxide synthase-like activity (NOS-like) was assayed as NO production detected by EPR in the presence of l-arginine and NADPH, and was 3.16+/-0.06 pmol/(mg FW min). The data presented here document the detectable presence of highly reactive species in M. arenaria.