The distribution in the dog prostate and other tissues of ciprofloxacin, a quinoline carboxylic acid derivative, was investigated in an experimental model. The concentrations in prostatic tissue, prostatic interstitial fluid (PIF), and prostatic secretion (PS) were lower than the corresponding plasma (P) concentrations, as would be expected for an acidic compound. The experiments were carried out under steady state conditions during intravenous infusion in one group of dogs and following gastric administration in another group. During steady state the ciprofloxacin concentrations were significantly higher in PS than in PIF, and the median PS/P ratios were significantly higher than the PIF/P ratios. These concentrations and ratios were compared with those of two other quinoline carboxylic acid derivatives, rosoxacin and norfloxacin. The concentrations of ciprofloxacin in prostatic tissue, PIF, PS, and urine were several times higher than the minimum inhibitory concentrations for most gram-negativve pathogen that cause bacterial prostatitis and urinary tract infections. Clinical trials of ciprofloxacin in these diseases are therefore indicated.