The urinary excretion kinetics of p-nitrophenol were studied in rabbits following oral administration of parathion at a dose of 3 mg/kg. Elimination of p-nitrophenol began rapidly, and of the total amount excreted during the study period, 46% was excreted in the first 3 h; 85% was excreted at 6 h after administration of the pesticide. The mean maximum excretion rate of p-nitrophenol was 111.15 +/- 61.02 micrograms/h reached in a time of 0.77 +/- 0.26 h. The formation and disappearance rate constants of the metabolite were 2.85 +/- 2.80 h-1 and 0.80 +/- 0.28 h-1, respectively. A linear relationship was observed between the plasma concentrations of parathion and the urinary excretion rate of p-nitrophenol.