Abstract: 1-(P-fluorophenyl) -6—fluoro-1,4-dihydro-4-oxo 7 (1-piperazinyl) cinnoline-3-carboxylic acid and its quinoline, 1,8-naphthyridine and pyrido (2,3-c) pyridazine analogues were prepared for studies of structure-activity relationships. The antibacterial activities of these 16 compounds were tested and the values of their electron densities were calculated using Hückel molecular orbital theory method. The results showed that the electron densities on the oxygen atoms of 3-carboxyl and 4-oxo have influenced strongly antibacterial activity in vitro. The quinolines andl,8-naphthyridines showed higher electron densities and more potent antibacterial activities in vitro. The einnolines and pyrido (2,3-c) pyridazines showed lower electron densities and lower or no antibacterial activities in vitro.