Ultrasonographic images of the accessory sex glands of 8 stallions were recorded immediately prior to sexual preparation, immediately after sexual preparation, and immediately after ejaculation. Relative size changes, determined by measurements of ultrasonograms of accessory sex gland, were contrasted. Length and width of the bulbourethral glands increased significantly (P less than 0.05) after sexual preparation and decreased significantly (P less than 0.05) after ejaculation. The increase in bulbourethral gland volume following sexual preparation was correlated significantly (P less than 0.01) with the number of false mounts attempted by stallions during sexual preparation. Diameter of the pelvic portion of the urethra did not change significantly (P greater than 0.05) after sexual preparation or after ejaculation. Prostate gland lobular and isthmic thickness increased significantly (P less than 0.05) after sexual preparation and decreased significantly (P less than 0.05) after ejaculation. Total and lumenal diameter of the ampullae increased significantly (P less than 0.05) after sexual preparation and decreased significantly (P less than 0.05) after ejaculation. Diameter of the ampulla wall did not change significantly (P greater than 0.05) after sexual preparation or ejaculation. Reduction of the lumenal area of the ampullae after ejaculation was not correlated (P greater than 0.05) to concentration of spermatozoa or total spermatozoa per ejaculate. Total and lumenal diameters of the vesicular glands increased significantly (P less than 0.05) after sexual preparation and decreased significantly (P less than 0.05) after ejaculation. Diameter of the vesicular gland wall did not change significantly (P greater than 0.05) after sexual preparation or ejaculation.(ABSTRACT TRUNCATED AT 250 WORDS)