An unconditioned response is an unlearned, naturally occurring reaction to an unconditioned stimulus. It is the automatic behavior that classical conditioning later attaches to a new cue.

Salivation in response to food in the mouth is the classic example. The response requires no prior training; it is part of the organism’s biological repertoire. Other unconditioned responses include the eyeblink to a puff of air, the withdrawal from pain, and the startle response to a sudden loud noise. In conditioning experiments the unconditioned response provides the baseline reaction that the conditioned stimulus will come to evoke.

The strength and form of the unconditioned response can be influenced by motivational state, fatigue, and drugs, yet its essential character remains unlearned. Measuring the unconditioned response allows researchers to confirm that the unconditioned stimulus is effective and to compare it with the later conditioned response.

The reaction that occurs naturally, without training, is the unconditioned response.