Mass spectra of a number of phenothiazines have been obtained by negative chemical ionization and compared with the corresponding mass spectra by positive chemical ionization and conventional electron impact. NH3, CH4, and N2O were used as reagent gases for negative chemical ionization, iso-C4H10 and CH4 for positive chemical ionization. The negative chemical ionization technique gives significant information regarding the molecular weight and the ring system of this group of compounds. As a complementary method to positive electron impact, negative chemical ionization is more informative than positive chemical ionization. Phenothiazines with two, but not three, methylene groups between the ring and the sidechain nitrogens given an intense rearrangement anion due to loss of the substituted nitrogen in the sidechain and two additional hydrogens. A 5-membered ring structure is postulated for this anion, which is formed with the reagents NH3 and CH4. but not with N2O. Apart from this peculiarity, the negative chemical ionization spectra are easy to understand. With NH3 as reagent, the anions representing the ring system and [M -- 1]- dominate the anion current. With CH4, both quasimolecular ions [M + 1]- and [M -- 1]- can be detected along with the much more intense ring anion. The reagent N2O yields cluster anions by oxidation of the ring and the molecule with the negatively charged oxygen formed in the ion source.