Objectives: The aim of the study was to evaluate histamine concentrations in plasma and tissues of breast cancers depending on the activity of histamine metabolic enzymes in neoplasmatic tissues of the breast gland. Material and Methods: In 95 women aged 38-70 years the concentration of histamine in the plasma by the immunoenzymatic method, the concentration of histamine in breast cancer tissues and metabolism enzymes of histamine: histidine decarboxylase, decarboxylase of aromatic L-amino acid, N-hista-mine methyltransferase, monoamine oxydase B, diamine oxydase determined using an isotope technique were assessed. The 24-hour excretion of N-methylimidazolacetate acid was evaluated by the chromatography method. Results: Significant increases were found of histamine concentrations in plasma tissues of ductal breast cancers, activity of histidine decarboxylase, aromatic L-amino acid and histamine methyltransferase. Conclusions: 1. Concentrations of histamine in plasma is dependent on the concentration of histamine in the tissues of ductal breast cancers. 2. Significant increases of histamine in cancerous tissues of ductal breast cancer could suggest the participation of this monoamine in the development of breast cancer. 3. The increase of histamine concentrations in ductal breast cancer tissues can be connected with disturbances in the balance between synthesis and enzymatic activation of this monoamine. 4. The concentration of histamine in plasma of women with ductal breast cancers is dependent on the number of lymph nodes and grade of histological malignancy.