We have determined the cellular concentration of thyroxine (T4) and triiodothyronine (T3) and the activities of two brain iodothyronine deiodinases, type II (5'-D2) and type III (5-D3), in two types of tissues --tumour (26) and non-tumour (5), derived either from human gliomas with various histological malignancies or from non-tumoural surrounding brain tissue. As it has been established, all patients before the surgery had the Non-Thyroidal Illness Syndrome (NTIS). The concentration of serum T3 was therefore significantly decreased in all the examined patients. It was over 2.5 times lower than that before surgery and 4.0 times lower at surgery than that seen in healthy controls. The serum concentration of T4 was found to be below normal range in 4/26 cases and in low levels of normal range in 6/26 cases, whereas TSH serum concentration in all patients was within normal range. The concentrations of T3 and T4 (expressed as pg of hormone/mg tissues protein) in 22/26 brain tissue samples were significantly lower in gliomas than in 5 non-tumoural brain tissue samples. As expected, the alternation in brain 5'D II activity in gliomas was seen in most cases with astrocytomas (5/8 cases), gliosarcomas (8/8 cases) and glioblastoma multiforme (10/10 cases). In general, the mean enzyme activity in tumour tissue was significantly higher than that found in non-tumoural tissue of human brain (21.79 fmol of newly generated T3/h/mg of protein vs. 4.88 fmol of T3/h/mg protein, respectively). The highest 5'D2 activity with a range from 10.82 to 45.96 (mean 23.61 fmol T3/h/mg protein) was found in gliosarcomas. The activity of 5-D3 was increased (in 8/8 cases of gliosarcoma and in 9/10 cases of glioblastoma multiforme) or decreased (in 3/3 cases of astrocytoma II, 5/5 cases of astrocytoma III) when compared to mean activity of this enzyme found in non-tumoural brain tissue. In summary, our results suggest that the concentration of brain iodothyronines and metabolism of thyroid hormones in the examined human brain tumours are altered. These changes may be related to malignant progression.