Basal levels of plasma cortisol in unstressed salmonid fish are normally in the range 0-5 ng ml(-1). An acute stress such as handling or 1 h confinement caused a temporary elevation of the plasma cortisol levels of both brown trout,Salmo trutta L., and rainbow trout,Salmo gairdneri Richardson, in the range 40-200 ng ml(-1) with a return to basal levels within 24-48 h. The extent of the cortisol elevation in response to an acute stress was dependent upon both the species and strain of trout. Chronic stresses, such as prolonged confinement or crowding, resulted in an elevation of plasma cortisol levels to approximately 10 ng ml(-1). Under these circumstances, blood cortisol levels remained elevated for periods of up to 4 weeks before acclimation finally occurred.It is shown, by means of intraperitoneal implantation of cortisol, that chronic elevation of plasma cortisol levels in the brown trout results in a dose-dependent increase in mortality due to common bacterial and fungal diseases. This effect is apparent at plasma cortisol levels as low as 10 ng ml(-1), levels below those often reported as being representative of 'unstressed' fish. These findings are discussed in relation to the known immunosuppressive effects of corticosteroids in teleost fish.