The Russian physiologist who discovered classical conditioning through experiments with dogs was Ivan Pavlov.
Pavlov initially studied digestion rather than learning. While measuring dogs' digestive secretions, he noticed that the animals began salivating before food arrived, sometimes when they saw the technicians who fed them. He investigated this anticipatory response systematically and found that a previously neutral signal, such as a sound, could become associated with food. After repeated pairings, the signal alone could trigger salivation.
This process became known as classical conditioning, also called Pavlovian or respondent conditioning. Food is an unconditioned stimulus because it naturally produces salivation; after learning, the paired signal becomes a conditioned stimulus, producing a conditioned response. The discovery helped establish important ideas about learned behavior and influenced psychology, education, and behavior therapy.
Pavlov is often remembered mainly for the dogs and the bell, although the famous bell is a simplified popular version of his work; some experiments used a metronome or other signals. His major scientific recognition came earlier: he won the 1904 Nobel Prize in Physiology or Medicine for research on digestion, not specifically for classical conditioning.