The aim of this study was to test whether conversion of xanthine dehydrogenase into xanthine oxidase as induced by fasting, ischemia of the liver or both is an in viuo process or only occurs in uitro in homogenates. For this purpose, the conversion rate of xanthine dehydrogenase into xanthine oxidase was studied in liver homogenates obtained from rats after normal feeding or 24 hr of fasting followed or not by 2 hr of ischemia of the liver. In fed rats, the conversion rate of xanthine dehydrogenase into xanthine oxidase was studied as well in liver homogenates after different periods of reperfusion after 2 hr of ischemia. Homogenization was carried out under strictly controlled conditions, after which the supernatants were incubated at 37" C in buffer for 0 to 5 hr. Enzyme activities were assayed spectrophotometrically by measuring urate production at 295 nm. Conversion started only after 2 to 3 hr of incubation of supernatants of control fed livers, whereas conversion started immediately after 24 hr of fasting. The percentage oxidase activity of total xanthine oxidoreductase activity in ischemic livers from fed animals was slightly higher (26.7% f 1.7%; p < 0.05) than in control livers (19.3% 2 1.4%), whereas the percent oxidase activity in ischemic livers from fasted animals (16.7% & 1.0%) was not different from that in control animals (16.8% f 1.1%). Ischemia for 2 hr caused in uitro a substantial increase in the conversion rate in supernatants of livers of fed and fasted rats as compared with their controls. Furthermore, the appearance of xanthine dehydrogenase and xanthine oxidase in the blood during reperfusion up to 60 min after 2 hr ischemia of the liver was studied. The enzyme started to appear in the blood after 5 min of reperfusion, predominantly as xanthine dehydrogenase. Very rapid conversion of xanthine dehydrogenase into xanthine oxidase was observed in the plasma. We conclude that fasting and 2-hr in vivo ischemia of rat liver can affect the conversion rate of xanthine dehy-