The lying behavior of Thoroughbred foals on breeding farms was continuously measured using triaxial accelerometers. Accelerometers were attached on the side of the hind limb cannon and under the halter of six foals to record triaxial angle data every 10 sec for a period of 24 hr. Lying behavior was divided into sternal lying and lateral lying based on head angle. Sampling was performed for two consecutive days each week until weaning. Sampling periods were divided into two periods on the basis of pasturing time: daytime pasturing (period A: 7-hr pasturing period, 2–60 days of age) and overnight pasturing (period B: 19-hr pasturing period, 32–152 days of age). Lying duration and frequency were longer and higher, respectively, in period A (44.6% of the time and 26.8 ± 7.4 times per day) than those in period B (27.7% of the time and 15.3 ± 4.2 times per day). In addition, foals spent more time laterally in period A (48.1% of total lying time) than in period B (38.9% of total lying time). Foals lie down longer in stalls than in pastures (Period A, 56.3 vs. 16.0%; Period B, 52.5 vs. 21.1%). Lateral lying was also longer in the stall than in the pasture. Lying duration and frequency changed with age in period A. Wet weather prevented lying behavior in the pasture. These results suggested that age, weather, and pasturing time affected the lying behavior of Thoroughbred foals under management at a breeding farm. Triaxial accelerometers may be useful for monitoring equine lying behavior.