Pavlov was investigating the gastric function of dogs, by externalising a salivary gland so he could collect, measure, and analyse the saliva produced in response to food under different conditions. He noticed that the dogs tended to salivate before food was actually delivered to their mouths, and set out to investigate this "psychic secretion", as he called it. He correctly decided that this was more interesting than the chemistry of saliva, and changed the focus of his research, carrying out a long series of experiments in which he manipulated the stimuli occurring before the presentation of food. He thereby established the basic laws for the establishment and extinction of what he called "conditional reflexes" - i.e. reflex responses, like salivation, that only occurred conditional upon specific previous experiences of the animal. These experiments were carried out in the 1890s and 1900s, and were known to western scientists through translations of individual accounts, but first became fully available in English in a book published in 1927.
Perhaps unfortunately, Pavlov's phrase "conditional reflex" was mistranslated from the Russian as "conditioned reflex", and other scientists reading his work concluded that since such reflexes were conditioned, they must be produced by a process called conditioning. As Pavlov's work became known in the West, particularly through the writings of John B. Watson, the idea of "conditioning" as an automatic form of learning became a key concept in the developing specialism of comparative psychology, and the general approach to psychology that underlay it, behaviorism. Bertrand Russell was an enthusastic advocate of the importance of Pavlov's work for philosophy of mind.
Unlike many pre-revolutionary scientists, Pavlov was highly regarded by the Soviet government, and was able to continue his researches until he was a considerable age. In later life he was particularly interested in trying to use conditioning to establish an experimental model of the induction of neurosis. His laboratory in Moscow has been carefully preserved.
It is popularly believed that Pavlov always signalled the occurrence of food by ringing a bell. In fact his writings record the use of a wide variety of auditory stimuli, including whistles, metronomes, tuning forks and the bubbling of air through water, in addition to a range of visual stimuli. When, in the 1990s, it became easier for Western scientists to visit Pavlov's laboratory in Moscow, no trace of a bell could be found.