The purposes of this study were to: 1) to examine the effects of time of day on salivary testosterone (sal-T) and salivary cortisol (sal-C) concentrations following acute bouts of high intensity interval training (HIIT) exercise performed in the morning and evening; and (2) to examine diurnal variations in the T:C ratio responses to HIIT exercise; and (3) to determine if there is a relationship between sal-T and sal-C responses to acute bouts of HIIT exercise and muscle mass. Ten men between the ages of 20-25 years who reside in the Norman or Oklahoma City Metro area were recruited for this study. Eligibility for participation was assessed using medical history and physical activity readiness questionnaire as well as completing the written informed consent and HIPAA agreement. Additionally, a dual energy x-ray absorptiometry (DXA) total body scan assessing bone-free lean body mass, fat mass, and percent body fat as well as a maximal graded exercise test to determine maximal oxygen uptake (VO2peak) was performed prior to participation in the intervention sessions. This randomized cross-over design assessment was aimed to determine whether or not HIIT exercise performed in the morning or evening effects sal-T and sal-C concentrations as well as determining if a relationship exists between bone-free lean body mass and the percent changes in the sal-T and sal-C responses. The findings from this study were: significant increases were observed for the sal-T exercise response (PRE to IP); significant decreases were observed for the sal-C time of day response (evening); and significant increases were observed for the T:C ratio time of day response (evening). In conclusion, sal-T and sal-C responses to acute bouts of HIIT exercise were not affected by time of day and no significant relationships existed between bone-free lean body mass variables and percent changes in sal-T and sal-C in this study.