A device for introducing substances inside animals includes:a base body for housing a cartridge containing a substance in the form of pellets to be introduced into an animala hollow needle for penetrating the animal and introducing the substancea push rod substantially aligned and coaxial with the hollow needlefirst operating elements for moving the push rod to push the substance through the hollow needlea tubular element fitted in the hollow needle and having temporary retention elements for temporarily retaining the substance inside the hollow needle and an open distal extremity of the tubular element, having: first and third segments with constant section, with transversal dimensions substantially equal to those of the tubular elementa second narrowing segment, associated with the first segment and having transversal dimensions at least in part smaller than those of the tubular element.
