1. Peritoneal mast cells from Brown-Norway (BN) rats were compared with those from Wistar and Sprague-Dawley (SD) rats. 2. Peritoneal mast cells from BN rats showed the smallest values in number, cell diameter and histamine contents compared with those from Wistar and SD rats. 3. BN rat peritoneal mast cells were more sensitive to compound 48/80 and anti-IgE than were those from Wistar and SD rats, and they showed a higher response to A23187 than did cells from Wistar rats. 4. The histamine release from passively sensitized peritoneal mast cells was weaker in BN rats than was that from Wistar and SD rats.