Eighty-seven male inmates from a state prison and 70 inmates from a county jail volunteered as subjects. The subjects, age 20 to 35 yrs, were assigned randomly into a control or exercise group. Their Vo2max and treadmill performance values were determined before and after a 20 week jogging program. Training intensity was between 85 and 90 percent of maximum heart rate and involved workouts 3 days/ week for 15, 30, or 45-min duration at the state prison and for 30-min 1, 3, or 5 days/week at the country jail. Cardiorespiratory fitness improved in direct proportion to frequency and duration of training. Injury, occurred in 22%, 24% and 54% of the 15, 30, and 45-min duration groups and in 0%, 12%, and 39% of the 1, 3, and 5-day/week groups, respectively. Attrition resulting from injury occurred in 0%, 0%, and 17% and in 0%, 4%, and 6% of the same respective groups. Attrition due to lack of interest was similar for all training groups (25%), but was significantly lower in the control groups (10%). Although the results showed a greater increase in cardiorespiratory fitness for the 45-min duration and 5-day/week groups, these programs are not recommended for beginning joggers because of the significantly greater percent of injuries.