The mechanism underlying the reaction time (RT2) slowing to the 2nd of 2 successively presented stimuli (S1 and S2) in the psychological refractory period paradigm was investigated. Stimulus onset synchrony (SOA) between S1 and S2, contrast of S2, and Task 2 set-level compatibility was manipulated. Specifically, the authors used a psychophysiological approach to examine RT2 slowing in trials in which the reaction time to S1 (RT1) was shorter than the SOA. For trials with RT1 less than SOA, the clear decrease in RT2 with increasing SOA was underadditive with the S2 contrast effect, but additive with compatibility. Electrophysiological measures revealed an exclusively premotoric locus of RT2 slowing. These findings indicate that a central bottleneck stage is occupied for some period after response to S1 execution, consistent with an extended response selection bottleneck account.