Brown trout caged in an acidic, softwater, Welsh hill stream running through close-canopy conifer forest and in an adjacent, circumneutral, moorland stream were studied after a natural rainfall event and after experimental acid and aluminium dosing of the forest stream. Endocrine (cortisol and thyroxine) and metabolic (glucose) stress responses occurred in fish held in the forest stream. Liming downstream of the acid zones mitigated these responses.