Antidepressants pose risks to aquatic animals and may affect fish behaviour. Toxicity information is needed to document biologically relevant endpoints. Circadian rhythms are under neuroendocrine control but have not been studied. Diurnal activity patterns were studied in fish exposed to SSRI and SNRI antidepressants. A mixture of fluoxetine, sertraline and venlafaxine upset these rhythms at 1 and 100 μg/L.