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The purpose of this study is to investigate the pharmacokinetics and tissue distribution of recombination bovine somatotropin (rBST) after subcutaneous administration of ^125I-rBST in male and female rats. A solid state conjugation (Iodo-bead) method was confirmed useful for producing ^125I-rBST because the administration of the conjugated form enabled enough to determine time-concentration relationships of rBST in rats. Subcuatenous administrations showed sex differences that female (t_1/2ka, 2.87h) revealed rapid elimination as compared to male ((t_1/2ka, 4.81h), with the absorption ((t_1/2ka, in male being 0.3 h and that in female 0.75 h) in the reverse order. For subcutaneous administration of rBST in male rats, the liver was the highest in amount, followed by kidney, testes, muscle, and stomach, at the slaughtering time of 1, 6, 12 and 24 h. But the testes was the highest at the 48h slaughtered animals, followed by liver, kidney, stomach, and muscle. In slaughtered females at 1, 6, and 12 h after the administration of rBST, the liver was the highest, followed by ovary, kidney, small intestine, and stomach. At 24 and 48 h slaughtered female rats, the ovary was the highest, the liver the second, and the kidney the third.
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