Using one force fed meal, absorption of labelled canthaxanthin 15,15'-3H2 was studied by collecting blood via caudal punction at regular intervals and measuring radioactivity in various tissues and organs of rainbow trout. 2. Canthaxanthin absorption showed a large variability between individuals irrespective of their sex. A rapid increase in the radioactivity linked to canthaxanthin 15,15'-3H2 is observed in the blood followed by a slow decrease. 3. Twenty-four and 72 hours after ingestion of labelled canthaxanthin by rainbow trout, the radioactivity was widely distributed. From the total radioactivity given, most was found 24 hours after the meal in the pyloric caeca, ovary and skin. Seventy-two hours after diet, the radioactivity increased in the muscle, liver and kidney. On the other hand, the percentage of radioactivity of intestine and caeca declined. 4. Data from absorption curve indicates that 0.86% of radioactivity ingested is present in the blood, suggesting that canthaxanthin is not readily transferred from the digestive tract. 5. The metabolic clearance of canthaxanthin (lambda = 0.0246/hr +/- 0.005) which would correspond to a half-life of 28 hours gives a preliminary idea of the digestive and metabolic utilization of canthaxanthin by the rainbow trout.
