In the late 19th century Ivan Pavlov and his lab team carried out several landmark surgeries on experimental animals, e.g. a surgery to introduce fistulas to the ducts of digestive glands, to isolate the gastric pouch in dogs as well as an esophagotomy carried out by Pavlov together with Ekaterina Shumova-Simanovskaya. The latter made possible the so-called “sham feeding” experiment which allowed the team to collect pure gastric juice unmixed with food. The team thoroughly studied its chemical composition and physiological properties, while further research showed a possibility to use the gastric juice from dogs in pharmacology to treat people with various gastric diseases. The gastric juice was first tried as a pharmacological medication at the therapeutic clinic of Military Medical Academy run by Sergey P. Botkin. The tests showed good results and an indisputable advantage of the natural gastric juice over pepsins available on sale. Soon, Ivan Pavlov’s laboratory started receiving orders. The preparation and sale of the new medication supervised by Evgeny Ganike, one of Pavlov’s employees, started in 1898. The medication was distributed to drugstores and clinics of Saint Petersburg, then other cities of Russia, and, before long, it was imported to Europe and, at last, America. The annual amount of juice production and sales grew steadily to reach 15,323 bottles by 1915. The proceeds from juice sales contributed to the laboratory development. After the 1917 revolution the production and sales of juice phased out as all the experimental dogs died due to the lack of special food. The paper features some texts and photos of the letters sent to Ivan Pavlov’s laboratory that have never been published before.