AbstractThe purpose of the present study was to define the elimination kinetics of phenazone (NFN) in the isolated perfused pig liver. In five experiments phenazone was administered as constant infusion to obtain steady‐state periods over a wide range of concentrations. The elimination of phenazone followed saturation kinetics (concentrations 0.1–12 μmol × 1‐1) and the maximal elimination rate (Vmax) was on average 102 μmol × min.‐1× kg‐1liver and the Michaëlis‐constant (Km) of 2.6 mmol × 1‐1. Estimates of Vmaxand Kmfor the microsomal phenazone hydroxylase activity measured in liver biopsies were found to be considerably lower than in the perfused liver. The hepatic elimination of phenazone during perfusion of pig liver at phenazone concentrations corresponding to human therapeutic doses follows