Ten Thoroughbred yearlings (5 females and 5 males) were used to examine the effect of time of grazing on pasture forage and digestible energy (DE) intake, bodyweight gain and DE expenditure in grazing activity. Five females were grazed for 17 h/day (LTG), 5 males were grazed for 7 h/day (STG) and they were fed differently. As a result, DE intake from pasture forage of LTG horses and STG horses was 27.3 and 12.7–13.9 Meal/day, respectively. The average daily gain (ADG) of LTG and STG horses was 0.37 and 0.39–0.61 kg/day, respectively. The regression lines between DE intake and ADG of both groups were parallel and the difference in DE intake between them was 10 Meal of DE. It was suggested that DE expenditure in grazing activity (entirely at night) of growing horses was 1 Mcal/h under the conditions of this study.