Phenacetin N-O-glucuronide is a semistable compound that breaks down slowly to form a reactive metabolite (half-life 8.7 hr). In phosphate buffer phenacetin-3-phosphate was formed and the amount of reactive metabolite formed was decreased. By studying the breakdown of phenacetin N-O-glucuronide in the presence of different phosphate concentrations and kinetically analyzing covalent binding, it was found there were two reactive metabolites. One reactive metabolite was inhibitable by phosphate and one was not. In the absence of protein both reactive metabolites could convert to acetaminophen, however, only one could form acetamide. The inhibition of microsomally formed reactive metabolites was similarly analyzed. Ascorbic acid could completely inhibit the reactive metabolite of acetaminophen but only partially inhibited the reactive metabolite of phenacetin.