Although dietary nucleotides have been determined to be required for normal immune function, there is limited direct interventional evidence confirming performance-enhancing effects of sublingual nucleotides in humans. A double-blind, placebo-controlled, randomized trial was conducted to evaluate the effect of sublingual nucleotides (50 mg/day) administered for 14 days in thirty young healthy physically active males, on endurance performance and immune responses. Fasting white blood cell count, natural killer cells (NKC) number, NKC cytotoxic activity, and serum immunoglobulin (IgA, IgM, IgG), and time to exhaustion, peak rate of perceived exertion, peak heart rate, and peak running speed during the exercise test were measured at baseline (day 0) and post-intervention (day 14). Time to exhaustion, as well as serum immunoglobulin A and NKC cytotoxic activity, were significantly higher at day 14 (p < 0.05) in participants supplemented with nucleotides compared with those who consumed placebo. No significant differences in other parameters were observed between groups at post-intervention. No volunteers withdrew before the end of the study nor reported any vexatious side effects of supplementation. The results of the present study suggest that sublingual nucleotides may provide pertinent benefit as both an ergogenic and immunostimulatory additive in active males.