The structural and functional characteristics of mast cells in the liver were investigated in alloxan diabetic rats after modulation of macrophage activity by aminophthalhydrazid (APH). The reparation of structural and metabolic alterations in the liver after administration of APH was accompanied by the change in mast cell functions. he morphometric analysis demonstrated increased secretion in mast cells, enhanced merocrine secretion and impaired degranulation after administration of APH.