In primidon-treated patients there are significantly decreased serum concentrations of total and free thyroxin, protein bound iodine and base line serum TSH values. In primidon-treated children T3-resin test values, concentration of thyroxin-binding protein and total cholesterol are identical to those of the control group. Degree of diminution in serum concentration of protein bound iodine, total and free thyroxin and base line TSH was independant of the primidon dose per day. Probably the demonstrated alteration in the thyroid function tests studied, is mainly caused by phenobarbital, the major metabolite of primidon and not directly by unmetabolized primidon. It is suggested that the high protein-binding capacity of phenobarbital results in a competitive displacement of protein bound thyroxin comparable to that of DPH. Phenobarbital is know to be a stimulator of the drug metabolizing enzyme system in the liver. This effect may be the cause of an increased turnover of T4 which results in a decreased serum concentration of total and free T4 at last. It seems possible that there is a balance in serum concentration of thyroid hormones on a lower level. Normal euthyroid state may be presumed, if T4-secretion raises, but there is no clue for an increased pituarity response. In contrast to the normal group in primidon-treated children the base line serum TSH values are decreased. It is supposed that another effect of primidon is responsible for this fact. There may be an influence of primidon treatment on hypothalamic pituarity axis. Our findings do not indicate clearly a hypothyroid state in primidon-treated patients; further investigations should give an answer to the guestion, if side effects as tiredness, decreased impetus and constipation are not partly caused by alterations in thyroid hormone system.