Distribution and Disappearance of Radioactivity in Blood and Tissues of Coho Salmon (Oncorhynchus kisutch) After Oral Administration of 3H-Testosterone

Yearling coho salmon (Oncorhynchus kisutch) were fed 3 times per day a diet containing 5 mg/kg of 3H-testosterone until the concentration of radioisotope in all tissues was constant. Radioactivity was monitored in blood and 16 tissues for 10 d after the hormone was withdrawn from the diet. Excretion was via the hepatobiliary pathway. Sixteen hours after the last feeding of 3H-testosterone, 93.1% of total radioactivity in soft tissues resided in the alimentary tract and gall bladder. Blood, skin, white muscle, and liver each contained about one fifth of remaining radioactivity. Concentrations of labeled substances in blood and testis were similar to or lower than normal androgen concentrations reported in salmonids. Ten days after steroid withdrawal concentrations were 1 ng/g or lower in all tissues.