The effects of feed intake, growth rate and temperature (8 and 12 degrees C) on apparent digestibility coefficients (ADC), blood uptake of individual astaxanthin E/Z isomers and metabolism of astaxanthin (3,3'-dihydroxy-beta,beta-carotene-4,4'-dione) were determined in Atlantic salmon. Accumulation of idoxanthin (3,4,3'-trihydroxy-beta,beta-carotene-4-one) in plasma was used to indicate metabolic transformation of astaxanthin. Quadruplicate groups of fish were subjected to three different treatments; one treatment was kept at 12 degrees C and fed to satiation. Another treatment kept at 12 degrees C was pair-fed with fish fed to satiation at 8 degrees C, resulting in a restricted feeding regime for the former treatment. After 2 months of feeding, the fish were fed a single meal containing ballotini glass beads to determine individual feed intake and Y(2)O(3) as an inert marker to determine ADCs. The faeces samples were pooled into 6 categories according to individual meal size (range 0.2-1.5% of body weight) and the ADCs for different meal sizes were determined. ADCs of astaxanthin ranged from 20% to 60% but were not significantly correlated with meal size. However, fish kept at 12 degrees C had approximately 10% higher ADC than fish kept at 8 degrees C (p=0.032). Growth rate and plasma astaxanthin concentration were higher at higher temperature and higher ration. Plasma concentration of idoxanthin was not affected by temperature or by meal size. The incidence of fin erosion and non-feeding individuals was significantly higher among fish fed a restricted ration indicating more aggressive interactions. Fish with visible fin damage had a tendency for having higher idoxanthin content in plasma than fish without noticeable fin damage. It is concluded that temperature but not individual meal size affect ADC of astaxanthin, whereas both influence plasma astaxanthin levels and may therefore affect the efficiency of astaxanthin utilization.