Aim. The aim of this study was to evaluate the efficacy of single ketamine infusion and clinical and biochemical factors connected with such efficacy, in patients with bipolar depression, which had not improved on antidepressant treatment. Methods. The study included 42 patients (32 women, 10 men), aged 22-67 years, with bipolar depression. They received ≥1 mood-stabilizing medications of first and/or second generation. After discontinuation of antidepressants (≥7 days), intravenous infusion of

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... ketamine (0.5mg/kg body weight) was performed. The assessment of depression by the 17-item Hamilton Depression Rating Scale was made before, and after 1, 3, 7 and 14 days following administration of ketamine. The assumed criterion for clinical improvement was the reduction of ≥50% score on the Hamilton scale after 7 days. In a subgroup of 20 patients, prior to administration of ketamine, serum concentrations of homocysteine, vitamin B12, folic acid, neurotrophins and inflammatory proteins were measured. Results. In the whole group, the severity of depression on the Hamilton scale decreased significantly 24 hours after administration of ketamine from 22.6±5.1 to 15.6±7.4 points. After 7 days it was 13±7 and after 14 days -11.8±7.8 points. Patients showing clinical improvement (n=22) had significantly higher frequency of alcohol addiction and family history of alcoholism. Biochemical tests in the subset of 20 patients demonstrated that those with clinical improvement (n=10) had higher serum concentrations of vitamin B12 and receptor-1 Vascular Endothelial Growth Factor before administration of ketamine. Ketamine infusion was well tolerated. Conclusions. The results confi rm a rapid antidepressant effect of ketamine infusion main-The results confirm a rapid antidepressant effect of ketamine infusion maintaining for 2 weeks, in a considerable proportion of patients with bipolar depression, and good clinical tolerance of such procedure. Also, some clinical and biochemical factors associated with ketamine efficacy were shown.