Respiratory pumping in the marine gastropod Aplysia is a well-characterized behavior controlled by identified neurons. The behavior is affected by stimuli such as change in ambient pH and shock. This study investigates learned changes in effects of these stimuli on rate of respiratory pumping movements. A sharp threshold exists for effects of environmental pH on respiratory pumping. Lowering the ambient pH from 7.8 to 7.0 does not affect the rate of respiratory pumping movements, but when pH is decreased further to 6.5 a large increase in pump rate is seen. Sensitizing stimuli, such as brief head shock and preexposure to pH 7.0, change the threshold so that respiratory pumping rate is increased in pH 7.0. Pairing exposure to pH 7.0 with head shock leads to pairing-specific amplification of the response in pH 7.0 alone. Pairing-specific consequences can be distinguished from sensitization only after an hour.