Differences in the glucose metabolism were examined and analysed in this study between patients treated with olanzapine and risperidone in comparison with healthy volunteers. The aim of the study was to determine differences of the impaired glucose metabolism in the study groups as well as to point out to the possible mechanisms which bring to these differences. To the group of 15 schizophrenic patients treated with olanzapine, and group of 15 schizophrenic patients treated with risperidone and to 14 healthy volunteers oral glucose tolerancy test is applied in order to determine the level of the impaired glucose tolerance. In the group of the patients treated with olanzapine glucose tolerance was impaired in 33% of the patients, while in the group of the patients treated with risperidone in 20%. Impaired glucose tolerance mostly manifested as hyperinsulinemia. Authors discussed about possible mechanisms responsible for the impaired glucose tolerance in the patients treated with new antipsychotics. Authors conclude that insulin resistance is the main mechanism for development of the diabetes type II in the schizophrenic patients treated with antipsychotics. Insulin resistance is the result of the multiple effects of the antipsychotics, among which most common are: increased body mass and direct involvement of the antipsychotics in the glucose metabolism.