The present study investigated the effect of target stimulus probability on the P3 component of the respiratory-related evoked potential (RREP). A single respiratory stimulus paradigm was employed where normal breaths served as standard stimuli and occluded breaths presented at various probability levels served as target stimuli. EEG was recorded from 29 channels in young adults in two conditions. In Condition 1, occlusions were presented at target probabilities of 0.5, 0.33, 0.25, 0.2, 0.1, and 0.05, that is, every 2nd, 3rd, 4th, 5th, 10th, and 20th breath. Order of target probability presentation varied randomly. Condition 2 used the same target probabilities, but held breathing rate constant by using a paced respiration task. Results indicated that P3 amplitude was significantly affected by stimulus probability, such that the component showed an inverse relationship with probability. This effect was similar between conditions. However, P3 amplitude reduced and latency increased when the secondary task of paced respiration was introduced. These data suggest that models developed to describe the effect of stimulus probability on the amplitude of visual and auditory P3 components are applicable to the respiratory somatosensory modality.