Abstract: The main objective of this study was first to study the normal concentration of blood Butirylcholinesterase in Nubian and Saanen Goats. To determine serum cholinesterase (ChE) a kinetic method was used. Normal mean values of ChE in 40 Nubian non lactating goats was found to be 0.031±0.008 U mL-1 in 4 lactating Nubian goats butirylcholinesterase values was 0.031±0.008 U mL-1 and in four Saanen goats 0.036±0.008 U mL-1. When 10 mg kg-1 of Fenthion was applied topically in the dorsal lumbar area, it was observed that serum ChE plummeted presenting a decrease of 64.8% during the first 3 to six days post application 64.5%. Thereafter, normal values similar to those observed before medication, were recorded until 18 and 24 days post application. None of the animals medicated showed signs of toxicity.