Protinases contained in the mid-gut of the early third instar of Gasterophilus intestinalis have been tentatively identified by midgut hydrolysis of synthetic substrates. Trypsin was identified by maximal hydrolysis of benzoyl-DL-arginine-p-nitroanilide (BApNA) at pH 8 and chym trypsin by maximal hydrolysis of benzpyl-L-tyrosine ethyl ester (BTEE) at pH 9. Carboxypeptidase A and B were identified by their maximal hydrolysis of hippuryl-DL-phenyl-lactic acid and hippuryl-L-arginine at pH 9 and 8 respectively. Aminopeptidase was identified by maximal hydrolysis of leucine-p-nitroanilide at pH 9. The mid-gut also showed activity of aspartic proteinase and identified it as cathepsin D. A drug (Banminth 12.5% pyrantel tartarate) used for the routine control of helminths parasite of horsed and donkeys in Egypt was used in vito to investigate its effect on the optimal activity of studied enzymes. It was found that the drug has no effect on trypsin and carboxypeptidase  while it decreases the activity of chymotrypsin, aminopeptidase and acidic proteinase and wa also found to increase the activity of carboxypeptidase B greatly.