Aim of Study: To demonstrate that administration of fluids and the consequent improvement of fluid balance during a surgical procedure can modify the tissue diffusion of ceftizoxime. Methods: Twenty-eight patients (30–79 years) undergoing major abdominal surgery of the colon were administered ceftizoxime 30 mg/kg i.v. at induction of anesthesia. A sample of arterial blood was taken before administration of the drug (t₀) and then again at the time of vascular occlusion of the colon segment to be removed (t1). A sample of the segment of removed colon was taken. The patients were divided into two groups on the basis of the fluid balance between t₀ and t1: group A (n = 17) with a fluid balance <1,000 ml and group B (n = 11) with a fluid balance >1,000 ml. The parameters evaluated in each group were: weight, height and age of the patients, serum and tissue antibiotic concentration, percent ratio of serum and tissue concentration, time elapsed between t₀ and t1, volume of administered fluids between t₀ and t1, diuresis and hourly diuresis between t₀ and t1 and body fluid distribution, obtained using a bioelectrical impedance analyzer. The mean results obtained in the two groups were then compared using Student’s t test. Results: The balance of fluids calculated up to t1 was 675 ± 308 ml for group A and 1,411 ± 405 ml for group B (p < 0.01). The means of the recorded values that showed statistically significant differences were: mean percent concentration ratio (43.6 ± 8.4 vs. 84 ± 16%; p < 0.05), concentration in the colonic segment (16.3 ± 7.9 vs. 37.2 ± 25.9 mg/ml; p < 0.05), urinary volume gathered up to t1 (538 ± 557 vs. 169 ± 104 ml; p < 0.05), hourly urinary volume up to t1 (311.1 ± 296 vs. 97.6 ± 77.9 ml/h; p < 0.05), percent variation of resistance (95.1 ± 5.1 vs. 89.7 ± 8.6; p < 0.05). The other means did not show any significant statistical differences. Conclusions: A higher tissue water level seems to facilitate the penetration of the antibiotic into the tissue according to the pharmacokinetic characteristics of ceftizoxime: high amount of free drug (not bound to plasma proteins) and high hydrosolubility.