The effects of iodine deficiency on the peripheral metabolism of thyroid hormone in immature rat were evaluated by measuring the kinetics of iodide and thyroxine (T4) in control and iodine-deficient 10-day-old rats. Iodine-deficient pups were obtained by giving the mother drinking water containing perchlorate; this anion is not transferred but prevents iodine transfer in the mother's milk. Labelled iodocompounds were measured in plasma, thyroid and skin for 48 h following intravenous injection of Na131I plus [125I]T4. Data were interpreted by compartmental analysis. The iodide plasma clearance rate, plasma equivalent distribution volume, plasma concentration, production and iodine thyroid content of iodine-deficient rats were significantly lower (-29%, -31%, -84%, -89% and -87% respectively) than in control 10-day-old rats. The iodide thyroid uptake was reduced (-47%) but remained higher than the release of iodine as T4. Cutaneous iodine was lost much more quickly by iodine-deficient pups than by control pups, explaining the decreased iodide distribution volume. The parameters of T4 metabolism were not changed by iodine deficiency, except for a slight but significant reduction of the thyroxinemia and T4 pools (-13%). Thence, T4 production was not significantly changed (about 8 pmol/h in control and iodine-deficient rats). The labelled T4 curves in iodine-deficient and control skin were superimposed and the patterns of labelled T3 derived from [125I]T4 were identical. Thus, the skin of immature rats converts T4 to T3; this process was not disturbed by iodine deficiency. The thyroid function of immature rats is particularly resistant to iodine deficiency, but the mechanisms remain unknown.