Seasonal variability in physiological parameters can be attributed to seasonal variations in environmental factors and/or to the consequence of the presence of endogenous circannual rhythms. In the current study we have measured plasma levels of lipids and of the different lipoprotein classes in fasting trout (Oncorhynchus mykiss) between the ages of 5 and 44 months. Independent of age and sexual maturity, a circannual variation in the low density lipoprotein concentration between 250 and 1300 mg/dl was demonstrated in both sexes. These seasonal fluctuations might be controlled by an endogenous biological clock synchronized by the photoperiod. The lipoprotein profile of trout is dominated by high density lipoproteins as early as the first months of life. Their concentration increases progressively during sexual maturation from about 1200 mg/dl in juveniles to about 2500 mg/dl during spermiation or at the moment of ovulation. This increase is highly significantly correlated with the increased concentration of testosterone occurring in both sexes during sexual maturation. The concentration of very low density lipoproteins increases substantially, from about 150 mg/dl to a maximal concentration of 800 mg/dl in females and 1100 mg/dl in males, during the deposit phase of lipid reserves which precedes the rapid increase in the gonadosomatic ratio. In the course of rapid ovarian growth, vitellogenin appears in the plasma of females and reaches a concentration of 2200 mg/dl 1 month before ovulation. From these results it is concluded that season and reproductive cycle are the two main factors affecting basal plasma lipid and lipoprotein levels in trout. Environmental factors such as photoperiod or endocrine factors such as the concentration of steroid hormones can be correlated and/or involved in the regulation of these quantitative variations. These results also suggest the presence of an endogenous biological clock able to exert an independent effect on plasma lipid and lipoprotein levels.