1307 To study the effects of long-term exercise and academic stress on gonadal steroids and bone density, 94 male (19±0.9 yr.) and 84 female (18±0.6 yr.) cadets at the United States Military Academy were evaluated every 5-7 months for 42 months. Values were compared among groups by ANOVA adjusted for repeated measures over time, (p<0.05). Both males and females had low gonadal steroid levels upon enrollment associated with the onset of an intense exercise program, and gains in bone mineral density (BMD) were not noted until the second year. In females (n=11) with persistently low estrodiol levels, 18±1.4 pg/ml compared to 65±2.8 pg/ml for the rest of the population, there was no gain in BMD over the course of the study, 1.22±0.032 to 1.26±0.036 g/cm2. In the other female subjects there was a significant increase in BMD from 1.27±0.011 to 1.31±0.014 g/cm2. In male subjects (n=12) with persistently low free and total testosterone (T) concentrations (Free T, 15±0.1 vs. 21±0.4 pg/ml; Total T, 297±14.1 vs. 466±9.9 ng/ml) BMD values, 1.21±0.039 g/cm2 initially and 1.29±0.035 g/cm2 at the end, remained lower than the rest of the male population. In the rest of the male population there was an increase in BMD from 1.31±0.014 to 1.38±0.014 g/cm2. Suppression of gonadal steroids with exercise/stress resulting in low BMD is usually associated with females, and was observed in the present study for subjects with estradiol levels <25 pg/ml. In addition we found intense exercise/stress resulting in persistently low testosterone levels in males to be a related to low BMD. Exercise/stress resulting in persistantly low gonadal steroid concentrations, over 42 months, is associated with reduced BMD irrespective of gender. Funded by U.S. Army MRDC grant #91MM-1502