Fifty Thoroughbred horses were examined. All horses had been in race training for a minimum of 4 months before examination and had worked at racing speed; 24 horses had raced. All horses were clinically sound at the time of examination. Ultrasonography was performed, using a 7.5-MHz transducer with built-in fluid offset. Videotaped images of the palmar soft tissue structures were obtained at 4, 8, 12, 16, 20 and 24 cm distal to the base of the accessory carpal bone (DACB). Images were digitized, and each image was calibrated. Values for cross-sectional area (CSA) and mean echogenicity (ME) were then determined from the cross-sectional images of the superficial digital flexor (SDF) and the deep digital flexor (DDF) tendons, using an image-analysis program. The SDF tendons were compared between right and left forelimbs at each level, and from proximal to distal on each limb, as were the DDF tendons. The relation between the SDF and DDF tendons for the same forelimb was determined at each level. There were no significant differences in CSA or ME at equivalent levels of the left and right SDF tendons. Mean (+/- SD) CSA was 1.01 +/- 0.12) cm2 at 4 cm DACB, 0.95 (+/- 0.14) cm2 at 12 cm DACB, and 1.12 (+/- 0.15) cm2 at 24 cm DACB. Adjusted ME was 2.34 (+/- 0.34) at 4 cm DACB, 2.03 (-/+- 0.38) at 12 cm DACB, and 2.04 (+/- 0.35) at 24 cm DACB. The left and right DDF tendons did not have significant differences in CSA or ME at any level. Cross-sectional area was 1.13 (+/- 0.18) cm2 at 4 cm DACB, 1.01 (+/- 0.12) cm2 at 12 cm DACB, and 1.75 (+/- 0.29) cm 2 at 24 cm DACB. Adjusted ME was 2.60 (+/-0.46) at 4 cm DACB, 2.49 (+/- 0.49) at 12 cm DACB, and 2.50 (+/- 0.44) at 24 cm DACB. At all levels, the left and right SDF tendons were smaller and less echoic than the DDF tendons of the same limb. The SDF and DDF tendons had an hour glass shape, with smallest CSA at 12 cm DACB. Mean echogenicity generally decreased for the SDF and DDF tendons from proximal to distal on the limb. These results indicate that for clinically normal trained Thoroughbred racehorses, there should be no significant difference in CSA or echogenicity between the left and right SDF tendons at equivalent distances DACB. There should be no significant differences in the left and right DDF tendons at equivalent levels DACB. The SDF tendon is usually smaller and less echoic than the corresponding DDF tendon at each level.