Morphometric methods were used to analyze the ultrastructural characteristics of peripheral blood polymorphonuclear neutrophils (PMN) in 10 rats chronically consuming ethanol and 20 rats fed an isoenergetic standard diet (10 ad libitum and 10 pair fed control rats). Morphometric measurements were made, after a 4-month experimental period, of the following: the profile area of the cell, nucleus and cytoplasm; nucleus to cell profile area; volume density of the nucleus, cytoplasm, mitochondria, Golgi system, endoplasmic reticulum and cytoplasmic granules; number of mitochondria per cell profile; number of cytoplasmic granules per cell profile and per micron2 of cytoplasm, as well as the azurophilic to specific granule ratio and mean diameter of granules. A significant decrease in cell profile area and cytoplasm profile area was shown in ethanol-treated rats. The volume density of mitochondria and endoplasmic reticulum nearly doubled during ethanol abuse. The results also showed that there were highly significant effects of ethanol on the total number of cytoplasmic granules per cell. In addition, changes were observed in mitochondria such as clumping, elongation, swelling and disruption of cristae, as well as changes in the topographic distribution of granules in the cytoplasm such as registration of cytoplasmic areas with numerous granules and areas with a smaller number or without any granules. Some neutrophils of ethanol-treated rats had autophagic vacuoles. The results indicate some ultrastructural abnormalities of PMN in chronic experimental alcoholism that may be related to polymorphonuclear phagocyte dysfunction.