The pharmacological interaction of the combination of Gentamicin sulphate and sulfamethoxazole-Trimethoprim was studied. An in vitro challenge method was used for this purpose. The results were used to calculate their interaction using the graphical representation known as ISOBOLOGRAM. The pure salts of Gentamicin sulphate and Sulfamethoxazole-Trimethoprim were used and were obtained from the manufacturer DIPROFA'Q Productos Farmaceuticos Veterinarios, Mexico. The agar bioassay method was used. The following criteria were used to quantify the results: A synergistic effect was defined when the actual/theoretical effect (SMEF) was greater than 1. The additive effect was defined when the SMEF was equal to 1. The area between the antagonism and additive effect was defined as the zone of indifference. When studying the interaction between Gentamicin and Sulfamethoxazole-Trimetroprim it was observed that the best SMF of this antibiotic combination was 2 : 3,. This result allows postulating that the use of this combination of Gentamicin and Sulfamethoxazole-Trimethoprim establishes a well recognised and documented synergistic effect between them and can be recommended in Canids and Felids for the treatment of enteric infections by gram + and gram- germs and other infectious agents sensitive to this combination.