The purposes of this investigation were to determine: 1) whether the fibrinolytic responses to acute, submaximal exercise were best related to intensity, duration, or total caloric expenditure; and 2) whether the time of day exercise is performed affects the fibrinolytic response. Twelve physically active men (mean age = 34.8 +/- 4.0 yr) performed four 30-min exercise sessions: 50% VO2max, a.m. and p.m., and 80% VO2max, a.m. and p.m. Blood samples were analyzed for tissue plasminogen activator (TPA) activity and plasminogen activator inhibitor-1 (PAI-1) activity. Data were analyzed using a three-way ANOVA with repeated measures. TPA activity: preexercise TPA did not differ among the four sessions. TPA increased with exercise in all sessions except the 50% a.m. session. Exercise at 80% increased TPA more than 50% (P < 0.001) and evening sessions increased TPA more than morning sessions (P < 0.05). PAI-1 activity: preexercise PAI-1 activity was higher during the morning than evening and significantly decreased with exercise in all sessions except the 50% p.m. session. It was concluded that changes in fibrinolytic activity appear to be influenced primarily by exercise intensity rather than duration or total caloric expenditure. Additionally, time of day of exercise performance significantly influenced fibrinolytic activity.