SummaryNorfloxacin nicotinate (NFN) is a new water‐soluble fluoroquinolone antibacterial agent. Thein vitroactivity of NFN for microorganisms isolated from swine and the pharmacokinetic properties of NFN following single intravenous (i. v.), intramuscular (i. m.) and subcutaneous (s. c.) administration were investigated.The minimal inhibitory concentrations (MIC's) of NFN for a wide range of reference (ATCC ‐ American Type Culture Collection) microbial swine isolates, comprising 21 bacterial and 5 mycoplasmal species, ranged between 0.03 μg/ml (forSalmonella cholerasuisandActinobacillus pleuropneumonia) and 12.5 μg/ml (forStreptococcus porcinus). The MIC of NFN forMycoplasma hyopneumoniae, the causative agent of enzootic pneumonia of pigs, was<1.0 μg/ml althoughM. hyosynoviaewas less sensitive, with a MIC value of 3.12 μg/ml. The MIC values of the drug for swine field isolates, comprising 8 bacterial species, were in good agreement with the values determined for the corresponding ATCC strains. Pharmacokinetic values for NFN were calculated following i. v. administration at 7.0 mg/kg and i. m. and s. c. administration at 14.0 mg/kg in a 3‐way cross over study involving 6 pigs. Plasma concentrations of unchanged drug were determined by HPLC during 24 h post injection. Plasma NFN concentrations measured after i. v. dosing best fitted a 2‐compartment open system pharmacokinetic model. The harmonic mean distribution half‐life (t1/2α) and elimination half‐life (t1/2β) were 4.2 minutes and 2.1 h, respectively. The mean residence time (MRT) was 2.9 ± 0.6 h and the steady state volume of distribution (Vss) was 3.2 ± 0.1 ***l/kg. The mean t1/2βvalues after either i. m. or s. c. administration were 4.45 h with very rapid absorption rates (<15 min to peak plasma drug concentration). Bioavailability was 51–64 %. Less than 20 % and 25 % of the dose were excreted in the urine as parent drug during the first 24 h after i. v. and s. c. dosing, respectively whereas the amount of unchanged drug recovered in the feces was very small (1.3 to 1.6 % of the dose).The pharmacokinetic and MIC data generated in the course of the present study suggest that a dose schedule of 14.0 mg/kg injected i. m. or s. c. every 24 h is capable of achieving and maintaining tissue drug concentrations of potential therapeutic value for the treatment of the most common bacterial and mycoplasm