The splitting of Sayphos into metabolites inhibiting indoxyl acetate esterase activity is investigated. Also the dynamics of Sayphos excretion in the urine is studied. It is established that in the urine of unpoisoned rabbits, normally a compound with Rf 0.79 is contained, inhibiting indoxyl acetate esterase activity. Sayphos and metabolite I with Rf 0.08 are demonstrable both in the blood and urine at 2 hours after peroral introduction of the poison. Within 24 hours no traces of the pesticide and metabolite I are discovered in the blood, and in the urine--within 96 hours since the beginning of the experiment. Most of the indoxyl acetate esterase activity inhibitors (Sayphos and metabolite I), excreted via urine (mean 95%), are emitted up to the 24th hour inclusive.