This study determined whether liking and relative reinforcing value (RRV) of physical activity were associated with time youth spend in moderate-to-vigorous physical activity (MVPA). Boys (n=21) and girls (n=15) age 8 to 12 years were measured for height, weight, aerobic fitness, liking and RRV of physical activity, and minutes in MVPA using accelerometers. Independence of liking and RRV of physical activity was established by a low correlation (r=0.08, p=0.64). Using MVPA as the dependent variable and hierarchical regression to control for individual differences in age, aerobic fitness, and time the accelerometer was worn in step 1 (R(2) of 0.60 for step 1), addition of liking and RRV of physical activity in step 2 produced an incremental increase in R(2) of 0.12 (p<0.01). When using median splits of the RRV and liking data to form subject groups, children with both a high liking and RRV of physical activity participated in greater (p< or = 0.02) MVPA (1340+/-70 min/week) than children with high RRV-low liking (1040+/-95 min/week), low RRV-high liking (978+/-88 min/week), or low RRV-low liking (1007+/-68 min/week) of physical activity. Thus, liking and RRV of physical activity are independently associated with MVPA. The combination of a high reinforcing value and liking of physical activity is associated with 33% greater participation in MVPA.