The purpose of this study was to investigate the effects of a plant supplement on aerobic and anaerobic performance, and over-training indicators in male long distance runners. "Marine AGX21" capsules, produced by Napro Pharma, Norway, contain Chromium Picolinate, Siberian Ginseng (powder and extract), Seaweed of the Fucus and Laminaria families (plant and extract) and Alfalfa. Two groups of eight athletes were randomly assigned, in a double-blind manner, to either supplement (mean age 32+8.7SD) of one capsule / 10kg body weight daily or placebo (age 34.4+7.4) for a period of 56 days. Variables were analysed using an ANCOVA to determine significant differences in scores over the supplementation period whilst adjusting for baseline measures. Pre supplementation and post supplementation measurements of weight (80.7+8.3 and 79.5+8.5kg respectively), body fat by skinfolds (16.8+3.5 and 16.5+3.3%) or bio-electrical impedence (16.6+4.3 and 16.7+3.7%), predicted VO2max (64.6+3.7 and 66.3+3.5mls/kg/min), anaerobic power (9.0+1.2 and 9.3+1.3watt/kg), and serum creatine phospho-kinase (CPK) (198.3+84.5 and 269.6+149.5IU/L) were not significantly different. However, HDL cholesterol increased significantly from 1.49mmol/L to 1.63mmol/L (p=0.004), serum testosterone increased from 15.6nmol/L to 17.93nmol/L (p=0.001) whilst serum cortisol decreased from 333.25nmol/L to 269.63nmol/L (p=0.036). No significant differences were determined for any placebo variables. Any training and dietary changes were monitored through individual diaries. A paired t-test showed no significant differences from pre to post test scores in either the placebo or treatment groups for miles run per week. It was concluded that "Marine AGX21" capsules did not enhance aerobic or anaerobic performance but may produce a positive ergogenic effect in male endurance athletes that could delay the onset of over-training symptoms. Decreased serum levels of testosterone and increased serum levels of cortisol are established indicators of over-training[1,2]. It is possible, therefore, that supplementation of "Marine AGX21" may reduce or prevent symptoms of over-training, or that supplementation might enable an athlete to train at higher and more intense levels without over-training syndrome occurring.