The aim of this work was to study the pharmacokinetic behaviour and the inhibitory effect on acetylcholinesterase and butyrylcholinesterase activities of chlorpyrifos in male and female cattle after pour-on administration. Determination of cholinesterase activity in plasma and erythrocyte was carried out according to Ellman kinetic method. The mean baseline activities were 9338.39 +/- 1331.61 and 13220.69 +/- 2274.18 to acetylcholinesterase and 624.65 +/- 39.32 and 641.68 +/- 88.08 IU/L to butyrylcholinesterase in females and males, respectively. Acetylcholinesterase was the predominant form of cholinesterase analyzed, with low levels of butyrylcholinesterase. The basal acetylcholinesterase activities of the bulls were significantly greater than those of cows. The inhibitory effect of topical chlorpyrifos administration was lower on butyrylcholinesterase than on acetylcholinesterase. Chlorpyrifos peak plasma concentration (male:10.920 +/- 4.18; female:12.12 +/- 3.88 microg/L) were reached at 11.92 +/- 9.19 and 8.17 +/- 7.67 h in male and female, respectively. The values of area under curve were 185.96 +/- 168.45 and 278.89 +/- 270.00 microg h/L and mean residence time were 13.95 +/- 8.10 and 14.90 +/- 9.80 h in male and female, respectively.