Although antibiotics frequently are added to the drinking water of mice, this practice has not been tested to confirm that antibiotics reach therapeutic concentrations in the plasma of treated mice. In the current investigation, we 1) tested the stability of enrofloxacin and doxycycline in the drinking water of adult, female C57BL/6 mice; 2) measured the mice's consumption of water treated with enrofloxacin, doxycycline, amoxicillin, or trimethoprim-sulfamethoxazole; and 3) used HPLC to measure plasma antibiotic concentrations in mice that had ingested treated water for 1 wk. Plasma concentrations of antibiotic were measured 1 h after the start of both the light and dark cycle. The main findings of the study were that both enrofloxacin and nonpharmaceutical, chemical-grade doxycycline remained relatively stable in water for 1 wk. In addition, mice consumed similar volumes of antibiotic-treated and untreated water. The highest plasma antibiotic concentrations measured were: enrofloxacin, 140.1 ± 10.4 ng/mL; doxycycline, 56.6 ± 12.5 ng/mL; amoxicillin, 299.2 ± 64.1 ng/mL; and trimethoprim-sulfamethoxazole, 5.9 ± 1.2 ng/mL. Despite the stability of the antibiotics in the water and predictable water consumption by mice, the plasma antibiotic concentrations were well below the concentrations required for efficacy against bacterial pathogens, except for those pathogens that are exquisitely sensitive to the antibiotic. The findings of this investigation prompt questions regarding the rationale of the contemporary practice of adding antibiotics to the drinking water of mice for systemic antibacterial treatments.