Recently, the topical application of a carnosine gel has been investigated as a possible ergogenic aid and may be beneficial for improving high-intensity exercise performance in team sport athletes. However, there is currently no research on the effects of a topical carnosine gel on performance in rugby sevens players. This thesis aims to: (1) review the current literature regarding carnosine, its importance for high-intensity exercise, the supplements used to increase carnosine concentrations, and the topical application of a carnosine gel (Chapter One); (2) investigate the application of a topical carnosine gel and its effects on intermittent high- intensity exercise performance in Olympic-level rugby sevens players (Chapter Two); and (3) summarise the finding of the thesis, discuss the strength and weaknesses and recommend future research directions (Chapter Three). In Chapter One, the literature on carnosine, its importance for high-intensity exercise, the supplements used to increase carnosine concentrations and the novel topical application of carnosine was reviewed. Carnosine has been reported to increase high-intensity exercise performance by enhancing intramuscular buffering capacity, regulating myosin ATPase, and increasing calcium sensitivity within the muscle. Nutritional supplements containing carnosine or β-alanine have been shown to increase intramuscular carnosine concentrations. However, these supplements often require a loading period of at least two weeks and often fail to substantially elevate plasma carnosine levels. Recently the topical application of a carnosine gel has been investigated as a possible ergogenic aid and may provide a more efficient method for increasing carnosine concentrations and improving high-intensity exercise performance in team sports, such as rugby sevens. In Chapter Two, eight Olympic-level rugby sevens players completed two performance tests in which they received both treatments, 10 mL of a topical carnosine gel (CAR) or an ultrasound placebo gel (PLA). The performance test was completed on a cycle ergometer and consisted of a warm-up, 12 intermittent sprints with a 2-minute half-time break. Each sprint effort consisted of 24 seconds cycling at 3 W/kg, 6 s at maximal intensity, and a 30 s rest. For peak power output the two-way ANOVA revealed a statistically significant time effect (p = 2.16 x 10^-9) and condition effect (p = 1.44 x 10^-7). The carnosine condition experienced an increase in peak power output for sprint 2 (+101.3 ± 81.0 Watts (W); p = 0.048; Cohen’s d = 0.99; large effect size), sprint 4 (+102.6 ± 82.1 W; p = 0.043; Cohen’s d = 0.74; moderate effect size) and sprint 7 (+ 156.0 ± 106.2 W; p = 0.025; Cohen’s d = 0.98; large effect size) compared to the placebo. Of note, all other sprints except for Sprint 12 were substantially greater in the CAR condition (d = 0.49 to 0.91). For mean power output, the two-way ANOVA revealed a statistically significant time effect (p = 5.98 x 10^-19), but no condition effect (p = 0.211). Although non- significant, there was still a substantial increase for mean power in the CAR condition in Sprints 10 (51.3 ± 48.2 W; p= 0.079; Cohen’s d= 0.57; moderate effect size) and 11 (44.7 ± 37.0 W; p= 0.069; Cohen’s d= 0.39; small effect size) compared to the placebo. These results suggest that the positive effects observed in this study are not limited to the typical intracellular buffering mechanism proposed for carnosine. In Chapter Three, the findings from Chapter Two were summarised and the conclusions of the thesis were presented. Overall, this thesis provides evidence for the efficacy of a topical carnosine gel for improving performance during repeated bouts of intermittent high-intensity exercise, especially in highly trained Olympic-level rugby sevens players. Further research should be conducted to investigate the possible mechanisms of action associated with the topical application of a carnosine gel in human athletes.