In Gram negative bacteria, fluoroquinolone resistance is acquired by target mutations in topoisomerase genes or by reducing the permeation of drugs due to the increase in expression of endogenous multidrug efflux pumps that expel structurally unrelated antimicrobial agents. An ongoing challenge is searching for new inhibitory substances in order to block efflux pumps and restore the antibiotic drugs susceptibility. In this research, we sought to investigate the interplay between ciprofloxacin and an efflux pump inhibitor (EPI), phenyl alanine arginyl beta-naphtylamide (PA beta N), to determine the prevalence of efflux pump overexpression in clinical isolates of Pseudomonas aeruginosa. Ciprofloxacin was tested at different concentrations (256-0.25 A mu g/ml) with a fixed concentration of PA beta N (50 A mu g/ml). The isolates susceptibility profiles were analyzed by disc diffusion and agar dilution methods using 10 antibiotic discs and 4 powders. It was found that in the presence of PA beta N, resistance to ciprofloxacin was inhibited obviously and MIC values were decreased. The comparison between subgroups of P. aeruginosa isolates with different resistance profiles indicates that efflux pump overexpression (EPO) is present in 35% of ciprofloxacin resistant isolates with no cross resistance and in variable frequencies among isolates showing cross resistance to other tested antibiotics: gentamicin (31%), ceftazidime (29%), and imipenem (18%). Altogether, these results imply that PA beta N maybe effective to restore the fluoroquinolone drugs susceptibility in clinical treatment procedures. Results also show that increased use of a fluoroquinolone drug such as ciprofloxacin can affect the susceptibility of P. aeruginosa to other different antipseudomonal agents.