To study the relative concentrations and appearance of immunoglobulin in the bladder and kidneys of rats with acute and chronic urinary tract infections, three groups of female Sprague-Dawley rats were studied. Group I underwent weekly intraurethral inoculation with Escherichia coli 07. Group II received inoculation of 0.2 ml of PBS alone. Group III were unmanipulated controls. Bladder and kidney washings were obtained for bacterial colony counts and organism identification. Bladder and kidney tissue was prepared for immunofluorescent examination using goat or rabbit antirat IgA, IgM, or IgG layered with fluorescent-labeled rabbit antigoat or goat antirabbit serum. Negative immunofluorescent controls substituted PBS for antirat serum. No significant difference was noted between bladder and kidney cultures in groups I and II. Comparison of immunofluorescent intensity between groups I and III demonstrated that IgM intensity was greatest between weeks one and three in bladders and kidneys, then decreased to non-significant levels. Bladder IgG intensity peaked at week four and remained elevated throughout the remainder of the study while renal IgG intensity was significant throughout the study period. Renal and bladder IgA was noted only sporadically. Comparison of culture positive animals to culture negative animals (groups II and III) demonstrated a lower immunofluorescent intensity score for culture negative animals. Discrete cellular immunofluorescence was evident in 19.1% of bladders and compared to 0.6% of kidneys. This study suggests that the immunologic response in the rat to intraurethral inoculation and infection is biphasic (IgM followed by IgG) and that a local cellular immunologic response may exist in the bladder.