Unilateral acute pyelonephritis were produced in rabbits by injecting E. coli, using the retrograde route and after a temporary ureteral obstruction. Animals were treated with ceftriaxone IM at a dose of 50 mg/kg/d and 100 mg/kg/d (corresponding to 1 g/d and 2 g/d in humans) either by a single daily injection or by two daily injections at 12th intervals. Sacrifices were performed at day 5 and 7 of therapy. Analysis of bacteriological, morphological and histological data obtained in infected kidneys demonstrates that the single daily injection of ceftriaxone is more effective than the same dose divided in two injections at 12 h intervals. These results are observed as well with low and high doses of ceftriaxone.