The plasma kinetics of parathion were studied in rabbits after i.v. administration of a dose of 1.5 mg/kg and oral administration of 3 mg/kg. The time course of parathion plasma levels administered intravenously followed a three-compartment kinetic model statistically, whereas when administration was oral, the optimum kinetic model proved to be two-compartmental. The process of the absorption of parathion is very fast with a mean value for the absorption constant (ka) of 33 +/- 15.41 h-1. The slow disposition half-lives for i.v. and oral administration had mean values of 5.08 +/- 3.08 and 1.08 +/- 0.27 h, respectively. From the values established for the parameters defining the distribution process the wide accessibility of parathion to the different body organs and tissues may be seen. Although the compound has a high elimination constant, this process is not limiting to distribution.