Objective: The purpose of this study was to determine whether 7 days of phosphate loading would augment the post-activation potentiation response of the leg extensors. Research design and methods: 14 individuals (9 males) had their left leg connected to percutaneous stimulation apparatus to evoke a maximum twitch force response. Following 2 minutes recovery subjects performed a 10 second maximum isometric voluntary contraction (MVC). Post-activation twitch responses were then elicited immediately (5 sec) and at 30-sec intervals for 5 minutes thereafter. After 30 minutes recovery subjects performed a 5 second maximal effort cycle test to determine peak power output. A 10 sec 2-leg MVC was then performed, immediately followed by another 5 second sprint and further sprints at one minute intervals for 4 minutes. This procedure was performed prior to any supplementation (CON) and following either a 7 day sodium chloride placebo (6 g.d-1; PLA) or tribasic sodium phosphate (4 g.d-1; P) supplementation period, separated by 14 day. An intravenous blood sample was obtained prior to testing on all conditions for assessment of plasma inorganic phosphate (Pi) concentrations. Results: P supplementation increased mean plasma Pi content 24% above CON (ANOVA p<0.05), with no effect following PLA (3% increase; p>0.05). Pre-activation peak twitch and peak power were unaffected by either condition (p>0.05). The 10 sec MVC resulted in significant twitch force and peak power potentiation immediately post-MVC in all conditions, with no difference between the supplements (p>0.05). Post-activation twitch force at the final 5 minute time point remained significantly potentiated in the P group (14%; p<0.05), though had returned to within baseline in CON (6%) and PLA (5%). At the final sprint peak power had also fallen to within pre-activation values in CON (-0.6%) and PLA (0.7%) but remained significantly elevated in the P group (1.8%; p<0.05). Conclusions: 7 d phosphate supplementation prolongs the effects of post-activation potentiation of both isometric twitch force and peak power output at the 5 minute time point.