We examined the possible relationship between thymic mast cells and increased vascular permeability in the thymus of autoimmune NZBW F1mice. Light-microscopic observation of tissue sections showed that non-autoimmune BDF1mast cells increased with age. In contrast, autoimmune NZBW F1mast cells did not increase in the thymic parenchyma at the age of 9 weeks. However, NZBW F1mast cells resumed the age-associated increase from the age of 12 weeks and exceeded the number of BDF1mast cells at the age of 30 weeks. Blood histamine levels of 9-week-old NZBW F1mice were higher than those of BDF, mice of comparable age. Furthermore, peritoneal mast cells of NZB/W F1mice were more sensitive to compound 48/80 than those of BDF1mice. Increased blood histamine levels of NZBN F1mice seem to be due to the enhanced histamine release from mast cells. These results suggest a possible correlation between the high histamine levels by degranulation of mast cells and increased vascular permeability in the thymus of NZB/W F1mice.