To investigate the amount and strength of exercise by rearing horses under night-and- day grazing conditions, moving distance and speed of 41 Thoroughbred foals, 50 one-year horses and 23 broodmares were measured by portable GPS fitted on head-collars. The research was carried out on grazing pasture (1 to 3 ha) in a private breeding farm in July, September and October from 2006 to 2008. The horses were grazed 840 to 1,200 minutes from afternoon to next morning and their behavior was recorded at 2 or 5-min intervals to measure the percentages of grazing or resting behavior at pasture. The data were analyzed statistically by ANOVA and student t-test. Grazing time of foals was shorter than that of one-year horses and broodmares (56.7 vs. 64.7 and 68.6 %, P<0.05), while their resting time was longer than that of older horses (34.0 vs. 25.4 and 22.6 %, P<0.05). Total distance covered and distance through grazing of one-year horses was 14.2 and 10.3 km/d, which were longer than those of foals (10.4 and 7.2 km/d, P<0.05) and broodmares (11.5 and 7.6 km/d, P<0.05). The average speed through moving of foals and one-year horses was 1,021 and 1,155 m/hr, which were faster than that of broodmares (905 m/hr, P<0.05). Total time of running with trot (2 to 5 m/sec.) and canter (>5 m/sec.) by one-year horses was 393 and 86 sec., which were longer than those by foals (181 and 43 sec., P<0.05) and broodmares (275 and 14 sec., P<0.05). Total distance through running by one-year horses was longer than that by foals and broodmares (1,730 vs. 770 and 820 m/d, P<0.05). In conclusion, the moving distance of foals was similar to those of broodmares although the grazing time was shorter, and one-year horses took the largest and strongest exercise under night-and-day grazing conditions.