Summary Six pregnant sows were treated in early pregnancy, late pregnancy and during lactation. Marbofloxacin was administered (2mg/kg body weight) intravenously and orally. The active drug concentration in the plasma was quantitated by use of high performance liquid chromatography (HPLC). Pharmacokinetic parameters were calculated by use of statistical moments. In lactating animals, the concentrations in milk were also determined by HPLC. Mean elimination half‐life of the drug after oral administration was significantly shorter in lactating sows (5.74 h) than that of the early pregnancy group (10.09 h). Total body clearance was highest in the lactating sows (3.27 ml/minute · kg body weight). The volume of distribution was large in all physiological states studied indicating good tissue penetration. Bioavailability was about 80% in pregnant and lactating sows. Antimicrobial secretion in milk contributed greatly to marbofloxacin elimination. These results indicate an important influence of lactation on marbofloxacin pharmacokinetics in sows. Therefore, in such cases, marbofloxacin dose should be increased during lactation.