Some biochemical effects of the repeated administration to rabbits of DDT (5 mg/kg), carbaryl (5 mg/kg), and parathion (0.5 mg/kg) were studied during 222 days. No significant changes were found in the blood or urine levels of amino acids. The tubular phosphorus reabsorption remained unaltered. The blood coagulation time was considerably shortened in the treated animals. The urinary excretion of 5-hydroxy-3-indoleacetic acid (5-HIAA) and 4-hydroxy-3-methoxymandelic acid (VMA) was higher in treated animals as compared to that of controls, indicating an increased rate of metabolism of serotonin and catecholamines, respectively. This increased metabolism is in turn believed to be a consequence of increased biosynthesis and release of biogenic amines caused by a stress mechanism induced by the administered pesticides. The amounts of 5-HIAA and VMA returned to normal 45 days after cessation of treatment.