Quantitative structure-activity relationships (QSAR) of various 7-(3-substituted-azetidin-1-yl)-1-cyclopropyl-6, 8-difluoro-1, 4-dihydro-4-oxoquinoline-3-carboxylic acids, 14-25, were studied to clarify the structural requirements for 3-substituted azetidines to potentiate antibacterial activity. A good parabolic relationship seemed to exist between the relative mean antibacterial activity indices against five representative gram-negative bacteria, GNM, and the calculated hydrophobic parameters, CLOG P, of these molecules. The CLOG P value of the most potent derivative was predicted to be around 2.3. On the other hand, against five representative gram-positive bacteria, the relative mean antibacterial activity indices, GPM, remained high and rather constant regardless of structural variation in the azetidine moiety. In order to confirm these findings, the QSAR analysis was extended with success to the quinolonecarboxylic acids, 26-34, which bear various substituted pyrrolidine, piperazine and piperidine derivatives instead of azetidines. The findings showed that the introduction of any amide substituent group to these heterocyclic amine moieties would lead to marked decrease in GNM, whereas incorporation of some amino substituent groups at a position two or three carbons remote from the N-1 position resulted in great enhancement of GNM. As azetidine quinolones exhibited somewhat low in vivo antibacterial activities, possibly reflecting their lesser bioavailability, we finally selected 3-amino-4-methoxypyrrolidine as one of the most promising C-7 substituent groups based on our QSAR analysis.