P50 suppression refers to the amplitude-reduction of the P50 event related potential to the second (S2) relative to the first (S1) of identical auditory stimuli presented 500. ms apart. Theory suggests that refractory periods (RPs) and/or inhibitory inputs (II) underlie P50 suppression. The present study manipulated interval between stimulus pairs (IPI: 2, 8. s) and direction of participants' attention (Attention, Non-Attention) in order to determine which theory best explains P50 suppression. The rationale is that: 1/ RP and II predict opposite effects of manipulating the functionality of the mechanism responsible for S2P50 suppression (e.g. reducing function would increase S2P50 according to the II and decrease S2P50 according to the RP hypothesis); 2/ IPI2 (relative to IPI8) will reduce functionality of the mechanism responsible for S2P50 suppression, as it results in less recovery of (and a greater challenge to) that mechanism - RP would thus predict reduced S2P50, whereas II would predict enhanced S2P50 amplitude; and 3/ where the mechanism responsible for S2P50 suppression is challenged (i.e. at IPI2, due to insufficient recovery), Attention (relative to Non-Attention) will enhance functionality of this mechanism - RP would thus predict increased S2P50, whereas II would predict reduced S2P50 amplitude. In the Non-Attention paradigm, reducing IPI from 8 to 2. s tended to increase S2P50 amplitude (and consequently impaired P50 suppression), and in the 2. s IPI paradigm, directing attention towards the stimuli reduced S2P50 amplitude (and improved P50 suppression), with both effects supporting the II hypothesis only.