1. Pyrenean brown trout (Salmo trutta fario) from 200 to 1700 g were analysed in lots of 12 specimens in March, April, July and October 1988, and in January 1989. 2. Hormones (insulin, glucagon, cortisol) and metabolites (glucose, lactate, proteins, triglycerides) were determined in plasma. 3. Body parameters (length and weight) increased progressively, while plasma metabolites and hormones were more dependent on seasonal and sexual conditions. 4. Plasma insulin presented a maximum in spring (11 ng/ml) and a minimum in January (5 ng/ml). A clear relationship between insulin and annual growth rates was established. Plasma glucagon showed annual values between 700 and 1500 pg/ml, with significant seasonal variations. The molar ratio glucagon/insulin oscillated from 0.21 in January to 0.38 in July. 5. Plasma glucose variations agreed with those of pancreatic hormones, especially evident with glucagon levels. Proteins were more uniform throughout year, and plasma lipids were especially associated to the reproductive state in females.