This thesis investigated possible changes in performance after one month of vitamin D supplementation in individuals found to be vitamin D deficient or insufficient through blood spot testing. Thirty-two males, ages 18-32, participated. Each subject visited the lab three times in one-month, completing four performance tests each session, including an isometric mid-thigh pull and a vertical jump on a force plate, a isometric 90-degree elbow flexion test using a load cell, and a psychomotor vigilance test on a palm pilot. The initial lab included blood spot tests to find vitamin D levels. In a single blind manner, 16 subjects were assigned vitamin D and 16 the placebo. Repeated measures ANOVA analysis did not reveal any main effects for time (F=2.626, p=0.364), treatment (vitamin D3 vs placebo; F=1.282, p=0.999), or interaction effects for treatment by time (F=0.304, p=0.999) for maximum force production during an isometric mid-thigh pull. Repeated measures ANOVA analysis did not reveal any main effects for time (F=1.323, p=0.999), treatment (vitamin D3 vs placebo; F=0.510, p=0.999), or interaction effects for treatment by time (F= 1.625, p=0.860) for rate of force production during a vertical jump. Repeated measures ANOVA analysis did not reveal any main effects for time (F=0.194, p=0.999), treatment (vitamin D3 vs placebo; F=2.452, p=0.513), or interaction effects for treatment by time (F= 1.179, p=0.999) for maximal force production during a 90-degree isometric elbow flexion. Repeated measures ANOVA analysis did not reveal any main effects for time (F=1.710, p=0.804), treatment (vitamin D3 vs placebo; F=1.471, p=0.94), or interaction effects for treatment by time (F= 0.293, p=0.999) for mean reaction time to random stimuli during the psychomotor vigilance test. Repeated measures ANOVA analysis did not reveal any main effects for time (F=0.530, p=0.999), treatment (vitamin D3 vs placebo; F=0.141, p=0.999), or interaction effects for treatment by time (F=0.784 p=0.999) for incidence of minor lapses during the psychomotor vigilance test