After his Ph.D., Harthoorn went initially to Uganda, then to Kenya and Tanzania. There he studied the effects of various sedative drugs on wild African mammals and with a team invented the M-99 (etorphine hydrochloride) capture drug and refined the tranquilliser gun, or 'Capture gun', for darting animals. This was a breakthrough in animal transport, enabling the safe movement of rare animals to game sanctuaries from places in which they were at risk from poaching or urban development. The research effort and its eventual triumph is recorded in Dr Harthoorn's first book: "The Flying Syringe". Capture gun technology is today used extensively in wildlife reserves, urban animal management, criminal human capture and zoological gardens. Before the invention of this technology animals were usually caught manually by being rounded up, penned, and transported without sedation, resulting in the death from stress of many hundreds of animals.