Efficacy of single-dose and 3-day trimethoprim-sulfadiazine (TMS) and amikacin treatment regimens for induced Escherichia coli urinary tract infections (UTI) in dogs was evaluated. Using each regimen, effects of giving TMS combination or amikacin were compared in males and females, and the response of treated dogs was compared with that of nontreated controls. Response to treatment was evaluated, using results of quantitative urine cultures and urinalyses obtained on 4 occasions. Abacteriuria was identified by finding a lack of bacterial organisms in specimens collected for the initial and final posttherapy evaluations. Before treatments, magnitudes of bacteriuria were similar in all experimental groups, and UTI persisted in all nontreated dogs. Single-dose treatment regimens did not reliably eradicate UTI in males or females, whether amikacin or TMS was administered. Magnitude of bacteriuria often diminished immediately after single-dose treatment, and such reductions of bacteriuria persisted in 2 of 8 dogs. However, no male dogs and only 1 of 4 females became abacteriuric after a single-dose treatment regimen. The single female in which UTI was eradicated was treated with a single dose of amikacin. The 3-day TMS treatment regimen eradicated UTI in each of 4 females, but the 3-day amikacin treatment regimen resulted in abacteriuria in only 1 of 4 females. Three-day treatment regimens were not effective in male dogs, regardless of the antimicrobial drug used. Of the short-course treatments for canine UTI evaluated by this model, only 3-day TMS treatment of females was consistently effective.