We have recently reported on the activities of glucose-6-phosphate dehydrogenase (G6PD), hexokinase (HK) and carbonic anhydrase I (CA-I) and II (CA-II) isoenzymes obtained from erythrocytes of healthy subjects and untreated patients with hyperthyroid diseases. Also, erythrocyte zinc concentrations were measured. Red blood cell (RBC) zinc (Zn) concentration was measured by atomic absorption spectrophotometry. Activities of carbonic anhydrase II and I isoenzymes were determined with CO2-hydratase activity method by using selective inactivation with bromopyruvate. G6PD and HK enzyme activities were measured spectrophotometrically via absorbance change (at 340 nm) in NADPH formed as a result of the reactions catalysed by these enzymes. In statistical analysis of all these parameters, activity of CA-I was 4388 +/- 207 (EU/gHb) and 2881 +/- 869 (EU/gHb) in healthy and untreated hyperthyroid subjects, respectively. The activity values for CA-II were 5391 +/- 257 (EU/gHb) and 4688 +/- 12.6 (EU/gHb) in healthy and untreated hyperthyroid subjects. Glucose 6-phosphate dehydrogenase (G6PD) activity was 10.19 +/- 1.87 (EU/gHb) in healthy group and 4.92 +/- 2.49 (EU/gHb) in patient group. While hexokinase enzyme activity was 1.575 +/- 0.898 in healthy subjects, it was 0.651 +/- 0.418 (EU/gHb) in the patient group. While erythrocyte zinc concentration in the healthy subjects was 49.32 +/- 23.5 (mg/gHb), this concentration for patients with uncontrolled hyperthyroid diseases was significantly decreased to 29.62 +/- 4.26 (mg/gHb). As a conclusion, CA-I isoenzyme, G6PD, hexokinase activities and erythrocyte zinc concentration had decreased in untreated patients carrying hyperthyroid diseases as compared to those of the healthy subjects.