The influence of food restriction, applied from birth to 36 h post partum, on neonatal thyroid function was studied in newborn Limousin x Romanov lambs. The control animals (n = 18) had free access to the mother and suckled ad libitum. Restricted lambs (n = 16) were removed from the mother and received limited amounts of colostrum in proportion to birth weight; 8 lambs were supplemented with lactose (30 g/l of colostrum). Plasma glucose and free fatty acid levels were significantly lower in restricted lambs, whereas urea levels were elevated. Plasma cortisol concentrations in control animals decreased during the period studied, but rose during the first 8 h of life in restricted lambs. Lactose supplementation only partially restored glucose and urea levels. Food restriction induced considerable modifications in neonatal thyroid function. The postpartum rise in plasma thyroid-stimulating hormone (TSH), thyroxine (T4) and free T4 levels occurring in control lambs was inhibited in restricted animals. However, the surge in plasma triiodothyronine (T3) levels was not affected, suggesting that this change was not related to physiological neonatal TSH hypersecretion. Thereafter, thyroid hormone concentrations decreased sharply during food restriction, whereas reverse T3 levels remained higher than in the controls. In response to these T4 and T3 deficiencies, plasma TSH levels rose only in lactose-supplemented animals. In agreement with the significant modifications in the values of the T3/free T4 and reverse T3/free T4 ratios, the abrupt changes in T3 and reverse T3 levels suggest that food restriction affected the peripheral conversion of T4 into T3 and reverse T3.(ABSTRACT TRUNCATED AT 250 WORDS)