Narrator: Pavlov's aim was to discover what caused saliva to flow. He rerouted the salivary ducts to the outside of his dog's cheek so that he could connect and measure the spittle. Perhaps he thought the production of saliva might be the result of a fixed nervous reflex, like a knee jerk.

Pavlov: [speaking russian]

Narrator: After taking many measurements of spittle, he confirmed that the dogs drooled automatically when their tongues touched food. He called the response the salivation reflex.

Pavlov: [speaking russian]

Narrator: But his work started to run into trouble. As his dogs became familiar with the experimental routine, they started to fill their cheek tubes before Pavlov had a chance to stimulate their tongues. The dogs were learning to anticipate food.

Pavlov tried a new technique. He erected screens so that the dogs couldn't see what was going on. Before passing meat through the hatch, he introduced a stimulus that was totally unrelated to feeding— a ticking metronome.

At first, the dog dripped saliva into its cheek tube only when the food appeared. But after a number of trials, the dog began to connect the ticking with the arrival of meat. Soon, the sound alone made the dog drool. Eventually, the dog salivated as much to the ticking itself as it did originally to the presentation of food.

Pavlov: [speaking russian]

Narrator: He called this new response the conditioned reflex.

Pavlov: [speaking russian]

[music box playing]

Narrator: Whatever the stimulus, his dogs could soon be conditioned to produce saliva. Pavlov believed that he had discovered how animals learned, even in the wild.