Although P50 is described as a largely preattentive process, increasing evidence suggests that the psychological state of a participant may influence P50 and its suppression. A paired-stimulus paradigm was used to examine the contributions of variability in stimulus parameters and state factors, such as expectancy and vigilance, on P50. Results obtained from 34 healthy subjects indicate that stimulus intensity and background stimulus intensity influenced P50 amplitude whereas stimulus duration had no significant impact. Importantly, P50 suppression varied with fluctuations in P50 amplitude to the first stimulus, and both P50 and its suppression reflected possible declines in attention or vigilance over the course of the session. Findings from this study suggest that P50 is not entirely preattentional and may reflect the psychological state of a participant. Implications of these results for research with schizophrenia patients are discussed.