Methyl parathion (20 mg X kg -1 intravenously and orally) and parathion (5 mg X kg -1 intravenously and 10 mg X kg -1 orally) were given to non-anesthetized dogs and the serum concentrations were followed in function of time. For both substances a low bioavailability after oral administration was found. In other dogs radioactivity was followed in urine after oral and after intravenous administration of labeled methyl parathion or parathion. The results suggest a good gastro-intestinal absorption of the substances. In anesthetized dogs which were given methyl parathion or parathion intravenously, high hepatic extraction ratios were found, suggesting that the low systemic availability after oral administration can be explained by an important hepatic first-pass extraction. Binding of methyl parathion and parathion to dog serum, to human serum and to a solution of human albumin was determined with equilibrium dialysis. In both species a high binding (greater than 90%) was found for both substances and there was no concentration-dependency in the concentration range used (0.2-30 micrograms X ml -1). In man and in dog the serum protein binding of parathion was about 5% higher than that of methyl parathion.