We determined the levels of brain acetylcholinesterase (AChE) and tail butyrylcholinesterase (BChE) activities in tadpoles of Odontophrynus americanus exposed to a commercial formulation of fenitrothion. The mean brain AChE activities in the controls tadpoles varied from 6.91 to 6.39 micromol min(-1) mg(-1) protein, whereas tail BChE activities ranged among 0.26 to 0.17 micromol min(-1) mg(-1) protein; the two sublethal concentrations of fenitrothion assayed produced AChE and BChE inhibition (p < 0.01). Brain AChE recovered a substantial level of activity with a maximum of 93.2%; after the transference of tadpoles to a free-pesticide solution, whereas tail BChE recovery showed a smaller increase (39%) in the activity at 168 hr after to transference to clear water. According with our results, we suggest that tadpole's tail BChE presents higher sensibility than brain AChE.