The sonographic and Doppler features of acute spermatic cord torsion were investigated in eight dogs. Serial sonography following torsion demonstrated unambiguous abnormalities in all experimental subjects within one hour. The observed abnormalities included increase in size of the spermatic cord, testis, and epididymis, decrease in echogenicity of the testis and epididymis, and loss of Doppler signal from the cord. Sonographic abnormalities were clearly visible before any histologic change was detected and well in advance of the onset of cellular necrosis. Since every experimental subject had an abnormal testicle, the identification of a normal testicle in a clinical setting is felt to exclude torsion. Combined gray-scale sonographic and Doppler features appear promising in the diagnosis of acute spermatic cord torsion and its distinction from acute epididymo-orchitis.