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Classical antihistamines are potent rat mast cel degranulators active in the range of 0.1 to 2.0 mM at pH 8.4. No relationship between their potency as mast cell degranulators and their chemical structure nor their activity as inhibitors of peripheral H1 histamine receptors could be established. Halogenation of diphenhydramine and cylizine, but not of promethazine, increased their activity. The alkylamino side chain of phenothiazine does not seem to be essential for degranulation since toluidine blue and methylene blue, which lack it, are also active. Thionin, whose amino groups in the phenothiazine ring are not methylated, is inactive.