Astaxanthin enters circulation in salmonid fishes upon intraperitoneal injection (IP) of small doses. Blood uptake and tissue distribution of geometrical E/Z astaxanthin isomers were determined in tissues and plasma of duplicated groups of rainbow trout (Oncorhynchus mykiss, initial weight 550 g) some of which were administered high doses of astaxanthin by IP in a trial lasting for 8 weeks. Doses of 10 (IP10), 50 (IP50) or 100 mg (IP100) astaxanthin (Lucantin Pink, BASF, Germany), respectively, dispersed in phosphate buffered saline were tested in comparison with diets containing 10 (Control) or 60 (Fed 60) mg astaxanthin kg(-1). Astaxanthin concentrations in all examined tissues and plasma were significantly higher in IP50 and IP100 than in controls and Fed 60 (p<0.05). In IP50, 11 mg astaxanthin kg(-1) muscle was detected after 4 weeks, compared to 4 mg kg(-1) in rainbow trout fed 60 mg kg(-1). Concentrations up to 80 and 100 mg astaxanthin kg(-1) were detected in liver and kidney after IP, respectively, whereas fish only fed astaxanthin contained about 2 mg astaxanthin kg(-1). No increase in muscle astaxanthin concentration was found between 4 and 8 weeks in fish given IP, and the muscle astaxanthin concentration in IP50 and IP100 were similar. Muscle concentration and injected dose were curvilinearly correlated and the proportion of ingested dose retained by the muscle was negatively correlated with the amount of injected astaxanthin. Plasma and muscle concentrations of astaxanthin were highly correlated (p<0.0001). Astaxanthin Z-isomers accumulated selectively in the various tissues after IP, whereas all-E-astaxanthin was preferably absorbed into plasma when administered via the diet. There was a selective uptake of all-E-astaxanthin in the muscle of all fish. Mortality was not affected by treatment, but a dose-dependent reduction in SGR was evident after IP. In conclusion, a more rapid and higher uptake of astaxanthin in plasma, muscle, kidney and liver of rainbow trout takes place after IP compared to when astaxanthin is fed via the diet.