Sonographic and anatomic observations were made of the kidneys of 23 Thoroughbreds or Standardbreds. In an in vitro study of 16 horses, precise correlations were established between the gross anatomic features of the kidneys and their sonographic appearance in images obtained in dorsal, sagittal, transverse, and transverse oblique anatomic planes. The renal cortex had a uniformly mottled echogenicity, and the renal medulla was relatively hypoechogenic, compared with the cortex. Acoustic anisotropy was observed in the cortex and medulla of the cranial and caudal extremities of each kidney. The distinctive renal pelvis was seen in the transverse plane as an echogenic pair of diverging lines that lead to the crescent shaped renal crest in the lateral half of the kidney. In images made in the sagittal plane, the renal pelvis was seen as a pair of parallel echogenic lines separated by the moderately echogenic line of the renal crest. The terminal recesses were best seen in the transverse oblique views of each extremity, where they appeared as moderately echogenic lines in the medulla of the cranial and caudal extremities. The interlobar vessels were represented as irregular echogenic lines in the medulla, and the arcuate vessels were seen as echogenic points at the corticomedullary junction. At the hilus, the renal artery or its branches was located cranial to the renal vein, which in turn was cranial to the position of the proximal portion of the ureter. In an in vivo study of 7 horses, sonographic images of the right kidney were obtained in the sagittal, transverse, and transverse oblique anatomic planes in all horses, with the transducer positioned at the 15th, 16th, or 17th intercostal space; images in the dorsal plane were obtained, however, in only 3 of the horses. For the left kidney, sonographic images were obtained in each of the anatomic planes when the transducer was positioned at the 16th or 17th intercostal space or the paralumbar fossa.