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Normal and dexamethasone-treated guinea-pigs were injected intravenously with 50 ng 131I-labelled \g=a\1-24 adrenocorticotrophin (ACTH). The concentration of radioactivity in the adrenals of dexamethasone-treated animals was significantly greater than in the untreated group but no difference was observed among the other tissues studied. There was no change in the concentration of 125I-labelled insulin, 125I-labelled poly-L-tyrosine, 131I\x=req-\ labelled bovine serum albumin (BSA) or [131I]iodide in any tissue including the adrenal. Intra-adrenal radioactivity after the injection of 131I-labelled \g=a\1-24 ACTH into normal animals showed a peak approximately localized at the glomerular-fascicular border with a second peak in the zona reticularis. Dexamethasone treatment significantly enhanced the size of both peaks. There was no difference in the intra-adrenal distribution of 125I-labelled BSA, 125I-labelled insulin and 125I-labelled poly-L-tyrosine after dexamethasone administration. The effect of other proteins on the increased uptake of 131I-labelled \g=a\1-24 ACTH by the adrenal of dexamethasone-treated animals was also studied. Human luteinizing hormone (100 ng), human growth hormone (100 ng), poly-L-tyrosine (50 ng) and glucagon (10, 100 and 1000 ng) were without effect. Excess \g=a\1-24 ACTH (100\g=m\g) and insulin (100 and 1000 ng) reduced adrenal radioactivity to the concentrations found in control animals.

SUMMARY Intact, unanaesthetized guinea-pigs were injected intravenously with [131I]α1–24adrenocorticotrophin (ACTH) (50–70 ng). The concentrations of radioactivity found in the kidney were considerably higher than in other tissues. There was no evidence for preferential accumulation in the adrenal gland as a whole when compared with 125I-labelled bovine albumin ([125I]BSA), [125I]poly-l-tyrosine and [125I]iodide. The intra-adrenal distribution of 131I radioactivity showed one peak at the border between the zona glomerulosa and the z. fasciculata and another in the z. reticulosa. The intra-adrenal distribution of [125I]BSA and [125I]iodide differed from that of [131I]α1–24ACTH.

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