Since no data are available concerning thyroid hormone levels in Snell dwarf mice from birth on, a cross-sectional study was performed of L-thyroxine (T4) and L-triiodothyronine (T3) levels in blood or serum as a function of age of several litters, starting at birth. In normal Snell mice T4 levels in blood and serum are changing with age. T4 increases during the first 2 weeks of age and declines thereafter, until adult levels of about 50 nmol/l are reached at 21 days of age. Serum T3 values are in the range of 2-3 nmol/l. They do not show such an age-related pattern. From birth on in each litter there was a clear separation between animals with low T4 levels in blood and the others. This separation was possible at all subsequent days until 9 days of age, when dwarfs can be recognized by eye. Above that age the low T4 values were associated with dwarfism. This suggests that dwarfs are hypothyroid already at birth. Serum T3 in dwarfs falls below the normal range only after 4 weeks of age, resulting in a lower T4/T3 ratio than normal. The half life time of exogenous T4 in serum of dwarfs is in the range of 13-18 h and not different from normal. For T3 t1/2 is 9.5-11.1 h. Dwarf mice become euthyroid by treatment with 0.1 microgram T4 per day. 1 microgram T4 was needed to reach a physiological level of T3. These data suggest that the peripheral conversion of T4 to T3 is slower in dwarfs than in normals. Treatment with hGH, prolactin, glucagon, insulin, testosterone and oestradiol had no influence on serum T4. As expected TSH was stimulatory. Similar results were obtained for serum T3, with the exception of prolactin which caused slightly increased levels of serum T3.