This investigation is concerned with the natural variations in aryl hydrocarbon hydroxylase (AHH) activity of flounder (P1atichthys flesus L.) throughout the year. A general trend towards higher activity in males was observed. It became significant during gonadal maturation, a period during which the activity in females was inhibited. Addition of a-naphthoflavone inhibited AHH activity except in females with maturing gonads. Enzyme activity measured at the optimal temperature for incubation showed highest activity during spring followed by a significant decrease during summer. Activity increased again during autumn, followed by a second decrease in the winter season. When activity was calculated based on ambient water temperature at time of sampling, smaller fluctuations between different seasons were observed; the only significant variation was high activity in June. The results obtained indicate that AHH activity is affected by both exogenous and endogenous factors, which should be taken into consideration if AHH activity is used as a biological indicator of marine pollution effects.