ABSTRACT: The Severn Estuary and Bristol Channel form the largest estuary in the UK. The estuary receives organic contaminants, including polyaromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) from diverse sources. Although hepaticethoxyresorufin O-deethylase (EROD) activity has been widely used as a biomarker for organic contamination in fish, its activity can be affected by a number of seasonal and environmental factors. We therefore measured EROD activity in the liver ofEuropean eels Anguilla anguilla and flounders Pleuronectes flesus, 2 abundant species in the Severn Estuary. Fish were collected from the water-intake screens of 2 nuclear power stations, located at Oldbury-upon-Severn andHinkley Point, every 2 to 4 wk between March 1996 and February 1998. EROD activity in eels showed pronounced seasonal variation. Maximum activity (656 to 820 pmol min^-1 mg^-1 protein) occurred in the warmest summer months and lowestactivity (117 to 128 pmol min^-1 mg^-1 protein) in the coldest winter months. EROD activity was also elevated during the summer in flounders (470 to 650 pmol min^-1 mg^-1 protein) but the highest EROD activities (upto 1546 pmol min^-1 mg^-1 protein) occurred in the winter/spring when spawning occurs in this species. However, these elevations occurred even in immature 1+ and 2+ flounders, suggesting that some other physiological/endocrinologicalcycles relating to day length may also be involved. Intraperitoneal injection of benzo[a]pyrene resulted in increased EROD activity in both species to levels similar to the maximum observed in fish from the field. The results confirm thatpronounced variation in EROD activity is associated with season and age in eels and flounders and these factors should be considered when employing this biomarker in the field.