It is believed that a monoamine oxidase (MAO) inhibitor specific for MAO A, which is reversibly bound to this enzyme and displaceable by tyramine, will be an antidepressant which will not cause a rise in blood pressure when tyramine-containing foods are ingested. Some linear tricyclic compounds with a larger and a smaller group forming the central ring and with a lipophilic group ortho to the larger group (here mostly the SO2 function of phenoxathiin 10,10-dioxide) are reported to have the sought properties. Potency appears to require short length and relatively small cross section for the substituent. The 1-ethyl (13), 1-vinyl (22), 1-trifluoromethyl (27), and 1-iodo (76) phenoxathiin dioxides had the best profiles. Structure-activity relationships, syntheses, and a possible rationale for the selectivity of these compounds and related tricyclics are given. Compound 13 was selected for further development. A summary of pharmacological data for 13 is given.