While animal manures are generally considered in terms of their importance as a nutrient source for growing crops, they can also serve as a point of entry for manure-borne pathogens and veterinary drug residues into the agroecosystem. A variety of veterinary drugs are registered and routinely used in food animal production systems for therapeutic or growth promotion purposes. These antimicrobials can be excreted in biologically active forms in manure and may persist in the soil when manure is land-applied. We conducted a laboratory incubation study in which the effects of three veterinary antimicrobials were tested on soil bacteria metabolic function parameters. The antimicrobials sulfadimethoxine or tetracycline were added to fresh layer poultry manure and homogenized, and the ionophore rumensin was mixed with fresh dairy manure. The manures amended with the antimicrobials were applied at a typical agronomic rate to moist sieved silt loam soil, and incubated in covered beakers at ambient temperature for 7 weeks. Soil alone and manure-amended soil without antimicrobial addition served as controls. There were four rates of each antimicrobial and three replications. Soil subsamples were taken periodically and analyzed for soil respiration, nitrification, iron (III) reduction, and community-level physiological profiles. Soil respiration and nitrification rates were generally unaffected by the antimicrobials, with most of the inter-treatment differences due to type of amended manure. Iron reduction was significantly reduced (P=0.05) in the poultry manure-amended treatements by all rates of sulfadimethoxine through day 50, and through day 15 in the dairy manure treatments by rumensin, with the rumensin effect decreasing over time at lower rates of the antimicrobial. These results suggest that some veterinary antimicrobials in the soil system can alter microbial metabolism. Given the little that is currently understood about these effects, further investigation of the impact of antimicrobials on soil bacteria is called for.